:/
Again, this sounds great till you realize it is another premise that starts off with something different, and then will immediately end up in the same stuff. The major change to minor change. Yes, he now has a kid, but he still gets kicked out (instead of being forced to move in with Haruka), and moves in at the dorm, resulting in a minor change of the greeting between Naru and Keitaro. Instead of, you know, him moving in with Haruka when he started going to university so that Sarah could see her "mom." The premise also seems to imply that all the character growth will be off screen, so we just see a different version going to the Dorms.
There is also the fact that even if Seta wakes up in the middle of the fic, a four year coma isn't something that you just shrug off. That only happens in movies (which is a bad plot device all round), and bad soap operas (as opposed to good ones?). Killing off/removing a character to get them out of the way is a sign of a fairly weak premise; which can be fine, but you have to work harder to make it work. Now, a character death as a premise can be fine, if it is meant to show the ripple effects of the death, or if it is a central aspect of the idea. Your central idea is having Keitaro be a father figure for Sarah, or at least an older sibling figure for her. That's doable without character death, or the marriage of convenience. You would need to tweak the character interactions between the two, starting with some scene, where maybe something forces them to be closer. It's not impossible.
For an example of why the character death (and general removal) is a weak premise, look no further than the Vin/Tifa fics in the FF7 section (first thing that popped in my head). There is no good way to remove Cloud from the equation without him walking out on his own; Tifa is damn dedicated, so she isn't likely to turn her back on him unless he, again, walks out or is dead. The point being, is that there is no good way to get around it so you smudge things a lot by getting rid of the main problem to the premise completely.
EDIT: Actually, the Vin/Tifa fic comparison is actually more similar than I realized at the time. The main ship fic, and the main ship author thelittletree, used Cloud walking out on Tifa as part of the basic premise. Eventually in the fic, Cloud comes back, but not before emotional ties are such that he seems out of place when he does. Had he come back too soon, everything would have gone back to the way it was before the story started. This is much the same, Seta wakes up/comes back during a time when the ties are just weak enough to make him being there kind of awkward, but just strong enough that he isn't likely to get back what he lost.
Again, this sounds great till you realize it is another premise that starts off with something different, and then will immediately end up in the same stuff. The major change to minor change. Yes, he now has a kid, but he still gets kicked out (instead of being forced to move in with Haruka), and moves in at the dorm, resulting in a minor change of the greeting between Naru and Keitaro. Instead of, you know, him moving in with Haruka when he started going to university so that Sarah could see her "mom." The premise also seems to imply that all the character growth will be off screen, so we just see a different version going to the Dorms.
There is also the fact that even if Seta wakes up in the middle of the fic, a four year coma isn't something that you just shrug off. That only happens in movies (which is a bad plot device all round), and bad soap operas (as opposed to good ones?). Killing off/removing a character to get them out of the way is a sign of a fairly weak premise; which can be fine, but you have to work harder to make it work. Now, a character death as a premise can be fine, if it is meant to show the ripple effects of the death, or if it is a central aspect of the idea. Your central idea is having Keitaro be a father figure for Sarah, or at least an older sibling figure for her. That's doable without character death, or the marriage of convenience. You would need to tweak the character interactions between the two, starting with some scene, where maybe something forces them to be closer. It's not impossible.
For an example of why the character death (and general removal) is a weak premise, look no further than the Vin/Tifa fics in the FF7 section (first thing that popped in my head). There is no good way to remove Cloud from the equation without him walking out on his own; Tifa is damn dedicated, so she isn't likely to turn her back on him unless he, again, walks out or is dead. The point being, is that there is no good way to get around it so you smudge things a lot by getting rid of the main problem to the premise completely.
EDIT: Actually, the Vin/Tifa fic comparison is actually more similar than I realized at the time. The main ship fic, and the main ship author thelittletree, used Cloud walking out on Tifa as part of the basic premise. Eventually in the fic, Cloud comes back, but not before emotional ties are such that he seems out of place when he does. Had he come back too soon, everything would have gone back to the way it was before the story started. This is much the same, Seta wakes up/comes back during a time when the ties are just weak enough to make him being there kind of awkward, but just strong enough that he isn't likely to get back what he lost.
