Bleach is a multiverse. That's canon fact. Different dimensions exist, entirely different worlds and universes exist, which Shinigami can access through the Precipice World and, presumably, police and patrol as much as they do the Material World that we see which Ichigo lives in. We don't ever see these worlds, but canon makes it clear that they do, in fact, exist.
Here's how the game works. We're going to wipe the slate clean, and then rebuild the upper echelons of the Court Guard Squad completely. All Captains, Lieutenants, Seated Officers, and the Captain-Commander are gone and do not exist. Instead, they will be replaced.
By who? Well, that's the name of the game. We're going to be making proposals and counter proposals as to exactly who would be best suited for the various openings in Soul Society.
Since Soul Society is a gigantic afterlife that sits in the nexus of an ambiguous multiverse, all settings are allowed to participate. Manga, anime, video games, TRPGs, books. There are no limits. The only requirements are that the individual must be (or have been at some point) powerful enough to fulfill whatever duties and obligations their position would require of them (i.e. proposing someone who has never been powerful and never noted as having the potential to become powerful as a Captain won't fly).
The only restriction is that absolutely no one from Bleach is allowed. The entire Court Guard as they exist in canon is gone. Abolished. Those characters do not exist, or if they do, they never became Shinigami/never rose above the rank and file Shinigami.
Basically, to condense the tl;dr, we're going to be building and posting our own 'lists' of who we think are best suited to be the Captains, Lieutenants, Seated Officers, and the Captain-Commander. Everything can be drawn upon to provide characters to propose, provided they meet the common sense standards that their proposed position would warrant (for instance, they have to be strong enough to do the job, the Gotai 13 is unlikely to allow straight-up villains to be Captains unless their benefits vastly outweigh their drawbacks, characters who are compulsive criminals are more likely to end up in the Maggot's Nest than get a promotion, ect, ect).
For the sake of deciding where someone might go and how powerful they might be:
Assume that dying and becoming a Death God has increased their fighting power, not decreased it, though by exactly how much is up to you.
Assume that any powers or abilities they possessed in life translate over into death (i.e. they can still use ki, they can still use magic, ect) in addition to acquiring the powers unique to Death Gods (the ability to wield "Demon Arts," i.e. Kido).
Assume that their Shikai (and Bankai, should they have one) reflects the powers and tools they possessed in life.
To help prevent mass Googling, I'm providing some basic information in the following quotebox that will help you build your lists. Remember: these lists do not have to be complete. That would be a whole hell of a lot of work, and this is supposed to be fun. Don't feel obligated to detail every Seated Officer, every Shikai, and every Bankai unless you want to. Your lists and proposals can be as detailed or as sparse as you want the to be. These are just some general points to help give your lists some direction and support.
If you want, you can read all of this and really put some effort into trying to propose a setup that makes sense and fits with everything. You can list Shikais, Bankais, power levels relative to others, and whatever else you want.
You could also ignore everything in the quote box and just go fuck it, it would be badass if these guys were Shinigami. That's fine too. It's up to you how detailed and in-depth your lists are or are not.
Holy shit that was long.
I'll start us off with my thoughts on the subject.
For Eleventh, I'm going to go ahead and say Guts would probably be the best Captain. He's the most Kenpachi-like person who isn't Kenpachi that I can think of, and is also likely one of the few characters in general who could really fill his shoes. As for power, Guts punches eldritch abominations to death and doesn't even really have any superpowers. Just giving Guts a semi-immortal body and the ability to move at speeds and strengths variable to his willpower would probably put him at Captain level, so I would say he definitely qualifies there.
For Third, I'm Kenshin all the way for Captain. He's not a psychopath like some people, but he's been through hell and back as an assassin. He understands more than most how sometimes certain lines have to be crossed, along with the pain of the prices that you have to pay for crossing them in the first place. I think he would really fit their ideal, and with his own skills combined with an immortal spirit body, he definitely would have the power to qualify as a Captain.
Some people might argue that Ryu Hayabusa would be the best fit for Second. And that is arguably true. But if we're going with the canon state of the Captain of the Second also being the Head of the Onmitsukid, I can really only think of one person who has the combination of: strength, ability to lead others, sheer charisma necessary to effectively control an entire military branch in addition to one of the Gotei divisions, and is also a ninja. I've gotta give this one to Naruto. Ryu has the skills to back up every last bit of his legend, but he works alone. He's not going to 'lead' anybody, much less an army of thousands. Plus, Naruto has, like, nine demon gods sealed inside him, and can blow up continents when he gets pissed off. Suck it, Soifon's Bankai! He's not even using his Zanpakuto yet!
Cross Marian is definitely a Captain somewhere, but the problem is, I'm not sure if he should go to Eighth "the party and womanizer" Division, or Twelfth "we'll science you to death" Division. He kind of fits both of them. In fact, he kind of fits both of them really freaking well. Having thought about it, though, he really fits Twelfth better, as his whole gig is that he's a super-scientist that has used magic and technology to push the boundaries of what an Exorcist can do, even going so far as to experiment on and turn Akuma. So Cross for Twelfth.
I'll think of more with time, but I'll leave my list here for now. The only other thing I can think to add at the moment is that there is only one single individual who I would trust with the power, authority, and responsibility of being Captain-Commander.
What's that? Hollows don't have Hearts? There's only one person for that job. . .
Here's how the game works. We're going to wipe the slate clean, and then rebuild the upper echelons of the Court Guard Squad completely. All Captains, Lieutenants, Seated Officers, and the Captain-Commander are gone and do not exist. Instead, they will be replaced.
By who? Well, that's the name of the game. We're going to be making proposals and counter proposals as to exactly who would be best suited for the various openings in Soul Society.
Since Soul Society is a gigantic afterlife that sits in the nexus of an ambiguous multiverse, all settings are allowed to participate. Manga, anime, video games, TRPGs, books. There are no limits. The only requirements are that the individual must be (or have been at some point) powerful enough to fulfill whatever duties and obligations their position would require of them (i.e. proposing someone who has never been powerful and never noted as having the potential to become powerful as a Captain won't fly).
The only restriction is that absolutely no one from Bleach is allowed. The entire Court Guard as they exist in canon is gone. Abolished. Those characters do not exist, or if they do, they never became Shinigami/never rose above the rank and file Shinigami.
Basically, to condense the tl;dr, we're going to be building and posting our own 'lists' of who we think are best suited to be the Captains, Lieutenants, Seated Officers, and the Captain-Commander. Everything can be drawn upon to provide characters to propose, provided they meet the common sense standards that their proposed position would warrant (for instance, they have to be strong enough to do the job, the Gotai 13 is unlikely to allow straight-up villains to be Captains unless their benefits vastly outweigh their drawbacks, characters who are compulsive criminals are more likely to end up in the Maggot's Nest than get a promotion, ect, ect).
For the sake of deciding where someone might go and how powerful they might be:
Assume that dying and becoming a Death God has increased their fighting power, not decreased it, though by exactly how much is up to you.
Assume that any powers or abilities they possessed in life translate over into death (i.e. they can still use ki, they can still use magic, ect) in addition to acquiring the powers unique to Death Gods (the ability to wield "Demon Arts," i.e. Kido).
Assume that their Shikai (and Bankai, should they have one) reflects the powers and tools they possessed in life.
To help prevent mass Googling, I'm providing some basic information in the following quotebox that will help you build your lists. Remember: these lists do not have to be complete. That would be a whole hell of a lot of work, and this is supposed to be fun. Don't feel obligated to detail every Seated Officer, every Shikai, and every Bankai unless you want to. Your lists and proposals can be as detailed or as sparse as you want the to be. These are just some general points to help give your lists some direction and support.
If you want, you can read all of this and really put some effort into trying to propose a setup that makes sense and fits with everything. You can list Shikais, Bankais, power levels relative to others, and whatever else you want.
You could also ignore everything in the quote box and just go fuck it, it would be badass if these guys were Shinigami. That's fine too. It's up to you how detailed and in-depth your lists are or are not.
Seated Officers are positions of power within a given division. There are a total of twenty Seats per division, resulting in a total of 260 total Seated Officers for the entire Court Guard. The First and Second Seats are the Captain and the Lieutenant, in that order. The remaining eighteen Officers in each division are individuals who have proven themselves to be of note within the division itself. Lower number Officers have higher authority over the division, including Officers who have larger numbers than themselves (i.e. the Third Seat has more authority than the Fifth Seat). The Captain, or First Seat, has supreme authority over their entire division. Within their division, their word is law.
It is generally understood that most, if not all, Seated Officers have at least Shikai. If not, then they would have to be exceedingly powerful and skilled to have attained Seated Status without actually possessing Initial Release.
Soul Society has it's forces divided into three divisions. The first overarching branch is the Gotai 13. The other two branches are the Kido Corps and the Onmitsukid (literally "Secret Force"). Not much has been canonically clarified about the Kido Corps and Onmitsukid. However, we do know that they seem to possess structures that are superficially similar to the standard Gotai divisions (i.e. there is a confirmed "Kido Captain" and "Kido Lieutenant"). We also know that one person can hold a position in both the Onmitsukid and another division (Soifon is the Captain of Second Division and the Head of the Onmitsukid). Given these facts, for the sake of building your lists, assume:
~ These two divisions exist separate from the Gotai 13 itself, but otherwise possess identical structures (twenty Seated Officers, with a Captain as First and a Lieutenant as Second).
~ The Kido Corp are a specialist division that focuses entirely on Kido (and for the purpose of this, Kido equivalents, such as magic). The Onmitsukid are Special Forces, spies, inquisitors, and assassins, the "interior" guard that is counterpoint to the Gotai 13's "exterior" guard.
~ Someone can be both the Onmitsukid Head and Captain of one of the Gotai 13 divisions, although this is not strictly required. To prevent confusion in the chain of command, assume that the lower Seats of the Onmitsukid do not overlap with other divisions (i.e. a Shinigami, even a Seated Officer, does not report to two separate Captains under any circumstances). In essence, it is possible for a single Captain to effectively head and belong to two divisions at the same time, but this only applies to that Captain. The actual divisions themselves do not overlap. In canon, this is actually considerably more complicated, because Onmitsukid were originally founded and run by the Shihon Noble Family, and the actual 'conjoining' of Onmitsukid Head and Captain only happened because Yoruichi, who was a Shihon, also became a Captain, thereby giving her de-facto power over both a Gotai division and the entirety of the Onmitsukid. This seems rather unbalancing and unfair, but that's politics, I suppose. It really speaks volumes of her character that she abandoned a position of power almost as great as the Captain-Commander just to follow Urahara and the Vaizard because their fates were unjust. However, that's complicated clan Noble Family stuff, and really doesn't have much place here, so for the sake of the lists, just assume that the Onmitsukid can be run either by someone who is solely the head of the Onmitsukid or by someone who is both the Head and the Captain of a Gotai division. It's easier that way.
tl;dr, all you really need to know is that, when a Shinigami graduates, they can choose to join the Gotai 13, the Kido Corp, or the Onmitsukid, with the latter two being identical in scope and stature to the Gotai 13 (so there's effectively three giant military branches in Soul Society, but the Gotai 13 are the most ovet and obvious).
Also, since we're on the subject, it should be noted that the Captain-Commander is the supreme commander of the Gotai 13, and not the other two divisions, who operate autonomously from the Gotai 13. However, it is possible for martial law to be declared in Soul Society (as it was when Central 46 was killed), thereby giving the Captain-Commander the power to directly control and coordinate all three branches of the military. This is part of the power and responsibility that comes with being the Captain-Commander, so try to keep that in mind before you decide to throw the job at Simon just because he can blow up galaxies by frowning.
And finally, the one part that is arguably the only useful thing in this entire quotebox, a general rundown of the themes of each of the thirteen divisions, to make it easier for you to match characters to where you think they should go.
First Division: No specific purpose or 'theme.' However, they are considered to be the elite of the Gotai 13, the very best of the best, so canonically, all the 'really' badass people are here (supposedly).
Second Division: Strictly speaking, we don't know this one, either. I'm tempted to go "ninjas" and just leave it at that, but technically speaking, that's the Onmitsukid's influence present. We don't know what the Division was about before Yoruichi was made Captain, but we do know that it's been stuffed with ninjas ever since. You can consider this the "ninja" division if you like, or you can call it a freebee and apply your own theme.
It should be noted that the Maggot's Nest is located within the Second Division area, so if you wanted to discard "ninjas" and keep that focused to the Onmitsukid, "jailers" or "guards/guardians/protectors" would be a fairly logical theme to go with.
Third Division: Third Division is the division of combat pragmatists. They favor 'honesty,' and put down that they 'hate liars' in their recruitment posters for Shinigami graduates. However, this apparently translates in reality into "hating people who have delusions about what battle really means." They prefer the truth. People are going to die, likely horribly. Neither you nor your friends are immune or immortal. Honor is for people too stupid or blind to recognize the usefulness of efficiently slitting someone's throat and moving on to the next target. A fight, a real fight, is kill or be killed, and if leaving our honor behind is what is necessary for more of our friends and allies to walk away alive, then that is a small price to pay. Ironically enough, Ichimaru Gin is actually the paragon poster boy of this philosophy, as he threw literally everything away (including potentially his life, though I don't know if that's been confirmed yet) just to get a shot at Aizen when he was at his weakest. If you want to stick with the canon theme, "people willing to pull a Gin" would be a pretty good place to start.
"Sneaky motherfuckers who don't play fair" would also be another good starting point for Third Division.
Fourth Division: The Healer division, Fourth devotes itself to being the doctors, nurses, and battlefield support for the rest of the Court Guard. The official recruitment posters note that they "don't mind anyone who wants to join," but do say that they prefer people who like helping others, have a strong aptitude for kido, and prefer to hang back and help other people instead of fighting on the front lines themselves.
Fifth Division: Sometimes viewed as the Kido division, Fifth is really themed along the lines of balanced fighters. They prefer recruits with a lot of potential and enthusiasm, and Fifth is generally regarded as the place where graduates and soldiers with a lot of talent go to have their skills and gifts refined. This is played out in canon, as three of the current Lieutenants and two Captains have associations with Fifth Division. Fifth favors skill, teamwork, and tactical skills, and seems to be the division that Shinigami with a lot of potential get sent to for refinement.
Of course, it could also be argued that the reason Fifth Division has that as it's theme is because Aizen wanted to ensure that any Shinigami with the potential to threaten him would at some point work under him, and thus be in a position to be exposed to his Shikai, so you could also argue that, since Aizen doesn't exist, Fifth Division is another 'freebee' division.
Sixth Division: Canonically headed by Byakuya, Sixth Division is made up of people who take 'duty' very seriously. There is a strong theme of traditional honor and upholding the virtues of order and justice in Sixth, which would suit people who tend to take social and personal rules very seriously.
Seventh Division: They believe strongly in duty and doing what is morally right. They follow a code of chivalry, and will gladly throw away their own lives and their Zanpakuto if it means serving a worthy cause or protecting something worth protecting. Slightly more free-thinking than some other divisions, the canon Captain, Komamura, encourages his subordinates to think for themselves and come to an understanding of what is right and what is wrong. Seventh favors hopefuls who are devout and skilled, and their recruitment poster says "we wait for those with unwavering courage." They also apparently pride themselves on physical strength, and have semi-annual strongman competitions.
Eighth Division: We know next to nothing about this division, beyond the fact that Shunsui and Nano belong to it. Shunsui insists that it's a carefree division and that they have frequent drinking parties, but Nano constantly contradicts him on that in the supplemental material. Taken at face value, it's a division that's a lot more loose and carefree than the others, with parties and drinking games going on during off-hours, though it could also be used as another freebee division, since we have so few strong facts about it.
Ninth Division: Tousen's former division, Ninth is in charge of security for the Gotei 13 and the rest of Soul Society, and as such, are technically in a constant state of standby. They also have strong ties to art and culture, and the Captain of the Ninth Division is the de-facto Editor-in-Chief of the Sereitei News Magazine and the Seireitei Communication newsletter. Ninth handles all the paperwork for the entire Serieti, and all reports and orders have to go through them at some point.
Tenth Division: Hitsugaya's division. Beyond his interactions with Matsumoto, we know absolutely nothing whatsoever about this division, and the two of them are such polar opposites that we can't even really discern a theme from looking at them, either. I'd consider this an empty slot, if only by virtue of the fact that the only detail we know about them is that "Tenth Division is a place that favors people who are kind." Which does kind of play out if you think about Hitsugaya and Matsumoto, but it's not much to go off of.
Eleventh Division: The division that needs no introduction, Eleventh is the "fuck kido we fight with swords and even our women have bigger balls than you" division. Kenpachi, obviously, is the canon Captain of this division. They look down on people who rely on Demon Arts and other, similar abilities to win fights, and love fighting for it's own sake.
Twelfth Division: Another division that likely needs no real introduction or clarification on my part. This is the Research and Development division. Urahara ran it in canon, and after him, Mayuri. SCIENCE~
Thirteenth Division: A kido-heavy division, potentially "the" kido division if you consider Fifth to not be that (and technically it is not), Thirteenth is fairly vague about what their theme is, but they do seem to heavily favor teamwork, kido, and people who are easy-going. If you're really strapped for themes, this is a potential freebee, though their obvious focus on kido does give them a lot more of a theme than some divisions.
It is generally understood that most, if not all, Seated Officers have at least Shikai. If not, then they would have to be exceedingly powerful and skilled to have attained Seated Status without actually possessing Initial Release.
Soul Society has it's forces divided into three divisions. The first overarching branch is the Gotai 13. The other two branches are the Kido Corps and the Onmitsukid (literally "Secret Force"). Not much has been canonically clarified about the Kido Corps and Onmitsukid. However, we do know that they seem to possess structures that are superficially similar to the standard Gotai divisions (i.e. there is a confirmed "Kido Captain" and "Kido Lieutenant"). We also know that one person can hold a position in both the Onmitsukid and another division (Soifon is the Captain of Second Division and the Head of the Onmitsukid). Given these facts, for the sake of building your lists, assume:
~ These two divisions exist separate from the Gotai 13 itself, but otherwise possess identical structures (twenty Seated Officers, with a Captain as First and a Lieutenant as Second).
~ The Kido Corp are a specialist division that focuses entirely on Kido (and for the purpose of this, Kido equivalents, such as magic). The Onmitsukid are Special Forces, spies, inquisitors, and assassins, the "interior" guard that is counterpoint to the Gotai 13's "exterior" guard.
~ Someone can be both the Onmitsukid Head and Captain of one of the Gotai 13 divisions, although this is not strictly required. To prevent confusion in the chain of command, assume that the lower Seats of the Onmitsukid do not overlap with other divisions (i.e. a Shinigami, even a Seated Officer, does not report to two separate Captains under any circumstances). In essence, it is possible for a single Captain to effectively head and belong to two divisions at the same time, but this only applies to that Captain. The actual divisions themselves do not overlap. In canon, this is actually considerably more complicated, because Onmitsukid were originally founded and run by the Shihon Noble Family, and the actual 'conjoining' of Onmitsukid Head and Captain only happened because Yoruichi, who was a Shihon, also became a Captain, thereby giving her de-facto power over both a Gotai division and the entirety of the Onmitsukid. This seems rather unbalancing and unfair, but that's politics, I suppose. It really speaks volumes of her character that she abandoned a position of power almost as great as the Captain-Commander just to follow Urahara and the Vaizard because their fates were unjust. However, that's complicated clan Noble Family stuff, and really doesn't have much place here, so for the sake of the lists, just assume that the Onmitsukid can be run either by someone who is solely the head of the Onmitsukid or by someone who is both the Head and the Captain of a Gotai division. It's easier that way.
tl;dr, all you really need to know is that, when a Shinigami graduates, they can choose to join the Gotai 13, the Kido Corp, or the Onmitsukid, with the latter two being identical in scope and stature to the Gotai 13 (so there's effectively three giant military branches in Soul Society, but the Gotai 13 are the most ovet and obvious).
Also, since we're on the subject, it should be noted that the Captain-Commander is the supreme commander of the Gotai 13, and not the other two divisions, who operate autonomously from the Gotai 13. However, it is possible for martial law to be declared in Soul Society (as it was when Central 46 was killed), thereby giving the Captain-Commander the power to directly control and coordinate all three branches of the military. This is part of the power and responsibility that comes with being the Captain-Commander, so try to keep that in mind before you decide to throw the job at Simon just because he can blow up galaxies by frowning.
And finally, the one part that is arguably the only useful thing in this entire quotebox, a general rundown of the themes of each of the thirteen divisions, to make it easier for you to match characters to where you think they should go.
First Division: No specific purpose or 'theme.' However, they are considered to be the elite of the Gotai 13, the very best of the best, so canonically, all the 'really' badass people are here (supposedly).
Second Division: Strictly speaking, we don't know this one, either. I'm tempted to go "ninjas" and just leave it at that, but technically speaking, that's the Onmitsukid's influence present. We don't know what the Division was about before Yoruichi was made Captain, but we do know that it's been stuffed with ninjas ever since. You can consider this the "ninja" division if you like, or you can call it a freebee and apply your own theme.
It should be noted that the Maggot's Nest is located within the Second Division area, so if you wanted to discard "ninjas" and keep that focused to the Onmitsukid, "jailers" or "guards/guardians/protectors" would be a fairly logical theme to go with.
Third Division: Third Division is the division of combat pragmatists. They favor 'honesty,' and put down that they 'hate liars' in their recruitment posters for Shinigami graduates. However, this apparently translates in reality into "hating people who have delusions about what battle really means." They prefer the truth. People are going to die, likely horribly. Neither you nor your friends are immune or immortal. Honor is for people too stupid or blind to recognize the usefulness of efficiently slitting someone's throat and moving on to the next target. A fight, a real fight, is kill or be killed, and if leaving our honor behind is what is necessary for more of our friends and allies to walk away alive, then that is a small price to pay. Ironically enough, Ichimaru Gin is actually the paragon poster boy of this philosophy, as he threw literally everything away (including potentially his life, though I don't know if that's been confirmed yet) just to get a shot at Aizen when he was at his weakest. If you want to stick with the canon theme, "people willing to pull a Gin" would be a pretty good place to start.
"Sneaky motherfuckers who don't play fair" would also be another good starting point for Third Division.
Fourth Division: The Healer division, Fourth devotes itself to being the doctors, nurses, and battlefield support for the rest of the Court Guard. The official recruitment posters note that they "don't mind anyone who wants to join," but do say that they prefer people who like helping others, have a strong aptitude for kido, and prefer to hang back and help other people instead of fighting on the front lines themselves.
Fifth Division: Sometimes viewed as the Kido division, Fifth is really themed along the lines of balanced fighters. They prefer recruits with a lot of potential and enthusiasm, and Fifth is generally regarded as the place where graduates and soldiers with a lot of talent go to have their skills and gifts refined. This is played out in canon, as three of the current Lieutenants and two Captains have associations with Fifth Division. Fifth favors skill, teamwork, and tactical skills, and seems to be the division that Shinigami with a lot of potential get sent to for refinement.
Of course, it could also be argued that the reason Fifth Division has that as it's theme is because Aizen wanted to ensure that any Shinigami with the potential to threaten him would at some point work under him, and thus be in a position to be exposed to his Shikai, so you could also argue that, since Aizen doesn't exist, Fifth Division is another 'freebee' division.
Sixth Division: Canonically headed by Byakuya, Sixth Division is made up of people who take 'duty' very seriously. There is a strong theme of traditional honor and upholding the virtues of order and justice in Sixth, which would suit people who tend to take social and personal rules very seriously.
Seventh Division: They believe strongly in duty and doing what is morally right. They follow a code of chivalry, and will gladly throw away their own lives and their Zanpakuto if it means serving a worthy cause or protecting something worth protecting. Slightly more free-thinking than some other divisions, the canon Captain, Komamura, encourages his subordinates to think for themselves and come to an understanding of what is right and what is wrong. Seventh favors hopefuls who are devout and skilled, and their recruitment poster says "we wait for those with unwavering courage." They also apparently pride themselves on physical strength, and have semi-annual strongman competitions.
Eighth Division: We know next to nothing about this division, beyond the fact that Shunsui and Nano belong to it. Shunsui insists that it's a carefree division and that they have frequent drinking parties, but Nano constantly contradicts him on that in the supplemental material. Taken at face value, it's a division that's a lot more loose and carefree than the others, with parties and drinking games going on during off-hours, though it could also be used as another freebee division, since we have so few strong facts about it.
Ninth Division: Tousen's former division, Ninth is in charge of security for the Gotei 13 and the rest of Soul Society, and as such, are technically in a constant state of standby. They also have strong ties to art and culture, and the Captain of the Ninth Division is the de-facto Editor-in-Chief of the Sereitei News Magazine and the Seireitei Communication newsletter. Ninth handles all the paperwork for the entire Serieti, and all reports and orders have to go through them at some point.
Tenth Division: Hitsugaya's division. Beyond his interactions with Matsumoto, we know absolutely nothing whatsoever about this division, and the two of them are such polar opposites that we can't even really discern a theme from looking at them, either. I'd consider this an empty slot, if only by virtue of the fact that the only detail we know about them is that "Tenth Division is a place that favors people who are kind." Which does kind of play out if you think about Hitsugaya and Matsumoto, but it's not much to go off of.
Eleventh Division: The division that needs no introduction, Eleventh is the "fuck kido we fight with swords and even our women have bigger balls than you" division. Kenpachi, obviously, is the canon Captain of this division. They look down on people who rely on Demon Arts and other, similar abilities to win fights, and love fighting for it's own sake.
Twelfth Division: Another division that likely needs no real introduction or clarification on my part. This is the Research and Development division. Urahara ran it in canon, and after him, Mayuri. SCIENCE~
Thirteenth Division: A kido-heavy division, potentially "the" kido division if you consider Fifth to not be that (and technically it is not), Thirteenth is fairly vague about what their theme is, but they do seem to heavily favor teamwork, kido, and people who are easy-going. If you're really strapped for themes, this is a potential freebee, though their obvious focus on kido does give them a lot more of a theme than some divisions.
I'll start us off with my thoughts on the subject.
For Eleventh, I'm going to go ahead and say Guts would probably be the best Captain. He's the most Kenpachi-like person who isn't Kenpachi that I can think of, and is also likely one of the few characters in general who could really fill his shoes. As for power, Guts punches eldritch abominations to death and doesn't even really have any superpowers. Just giving Guts a semi-immortal body and the ability to move at speeds and strengths variable to his willpower would probably put him at Captain level, so I would say he definitely qualifies there.
For Third, I'm Kenshin all the way for Captain. He's not a psychopath like some people, but he's been through hell and back as an assassin. He understands more than most how sometimes certain lines have to be crossed, along with the pain of the prices that you have to pay for crossing them in the first place. I think he would really fit their ideal, and with his own skills combined with an immortal spirit body, he definitely would have the power to qualify as a Captain.
Some people might argue that Ryu Hayabusa would be the best fit for Second. And that is arguably true. But if we're going with the canon state of the Captain of the Second also being the Head of the Onmitsukid, I can really only think of one person who has the combination of: strength, ability to lead others, sheer charisma necessary to effectively control an entire military branch in addition to one of the Gotei divisions, and is also a ninja. I've gotta give this one to Naruto. Ryu has the skills to back up every last bit of his legend, but he works alone. He's not going to 'lead' anybody, much less an army of thousands. Plus, Naruto has, like, nine demon gods sealed inside him, and can blow up continents when he gets pissed off. Suck it, Soifon's Bankai! He's not even using his Zanpakuto yet!
Cross Marian is definitely a Captain somewhere, but the problem is, I'm not sure if he should go to Eighth "the party and womanizer" Division, or Twelfth "we'll science you to death" Division. He kind of fits both of them. In fact, he kind of fits both of them really freaking well. Having thought about it, though, he really fits Twelfth better, as his whole gig is that he's a super-scientist that has used magic and technology to push the boundaries of what an Exorcist can do, even going so far as to experiment on and turn Akuma. So Cross for Twelfth.
I'll think of more with time, but I'll leave my list here for now. The only other thing I can think to add at the moment is that there is only one single individual who I would trust with the power, authority, and responsibility of being Captain-Commander.
What's that? Hollows don't have Hearts? There's only one person for that job. . .