Shiba Isshin was previous captain of the 10th Division with Shiba Kuukaku as his lieutenant. There was a confrontation with a Vasto Lorde twenty years before canon which resulted in the loss of KuukakuÆs arm and Isshin using up most of his power to defeat it, causing him to be stuck in the human world as he recovers, likely due to Garganta shenanigans. Regardless of the backstory, he ends up in the human world in a need of a gigai for power recovery.
Urahara finds him and offers him the user of a gigai which contained his Hogyoku. The gigai is designed to turn him into a human and lose his powers permanently to hide the Hogyoku forever. Thankfully, during his time in Karakura Isshin meets and falls in love with Kurosaki Masaki and wants to be a human. At that point Urahara reveals what he was planning and Isshin gives his consent and builds a life with Masaki.
But Urahara realises thereÆs a problem when Isshin ends up in a confrontation with an imperfect arrancar. He not only sees the arrancar but was able to kill it, something that should be impossible for a regular human. And after the battle, he really does appear to be a regular human. Checking on the Hogyoku, Urahara discovered that the barrier it was contained had eroded and it was bonding with IsshinÆs soul.
At this point, he either remakes the barriers whilst keeping it in IsshinÆs soul or he removes it in the hope that he can find another shinigami to use it on for the right result.
An important point to consider is the effect that this could have on IsshinÆs children, Ichigo in particular. Either the Hogyoku itself or IsshinÆs altered soul could changed the nature of his children; the Hogyoku is designed to dissolve the barrier between shinigami and hollows, so it could be that these children are born without that barrier in the first place. At the very least, Ichigo is affected since heÆs the protagonist.
Canonically, Ichigo is probably the closest to a perfect hybrid but this time he would be a natural born fusion of the races. Depending on the writer, this could easily be used to create a divergence during his childhood. Alternatively it wouldnÆt become significant until canon.
IchigoÆs power might follow the same route as the vizard or he could be a quasi-arrancar. Maybe he wonÆt even have an inner hollow since his powers all one thing this tiem. It could be a literal fusion where Ichigo can naturally use all the powers of shinigami and hollows/arrancar i.e. shikai, bankai, cero, resurreccÝon etc. ItÆs up to the writer though IÆm interested in the numerous possibilities.
Getting back to the plot, in the first scenario, everything that happens with Rukia is actually meant to be a set-up to trick Aizen into believing he had hidden the Hogyoku inside her. This undoubtedly screws up his plans when Aizen reveals himself when he discovers a) thereÆs nothing inside Rukia or B) a fake Hogyoku that might be a trap.
Aizen taking a fake Hogyoku to Hueco Mundo or being killed there and then could have a big impact on the upcoming events. In the latter case, Gin and Tousen are in the process of betrayal and thereÆs still a group of arrancar in Hueco Mundo. Though Stark wouldnÆt be one of the Espada and IÆm unsure which arrancars came about from the æperfectÆ Hogyoku.
In the second scenario, then Rukia really is the container and canon business goes down (or however the person writing the story sets it up). In this case, the most important factor of the idea is what the hell Ichigo is and how it might effect various plans.
Urahara finds him and offers him the user of a gigai which contained his Hogyoku. The gigai is designed to turn him into a human and lose his powers permanently to hide the Hogyoku forever. Thankfully, during his time in Karakura Isshin meets and falls in love with Kurosaki Masaki and wants to be a human. At that point Urahara reveals what he was planning and Isshin gives his consent and builds a life with Masaki.
But Urahara realises thereÆs a problem when Isshin ends up in a confrontation with an imperfect arrancar. He not only sees the arrancar but was able to kill it, something that should be impossible for a regular human. And after the battle, he really does appear to be a regular human. Checking on the Hogyoku, Urahara discovered that the barrier it was contained had eroded and it was bonding with IsshinÆs soul.
At this point, he either remakes the barriers whilst keeping it in IsshinÆs soul or he removes it in the hope that he can find another shinigami to use it on for the right result.
An important point to consider is the effect that this could have on IsshinÆs children, Ichigo in particular. Either the Hogyoku itself or IsshinÆs altered soul could changed the nature of his children; the Hogyoku is designed to dissolve the barrier between shinigami and hollows, so it could be that these children are born without that barrier in the first place. At the very least, Ichigo is affected since heÆs the protagonist.
Canonically, Ichigo is probably the closest to a perfect hybrid but this time he would be a natural born fusion of the races. Depending on the writer, this could easily be used to create a divergence during his childhood. Alternatively it wouldnÆt become significant until canon.
IchigoÆs power might follow the same route as the vizard or he could be a quasi-arrancar. Maybe he wonÆt even have an inner hollow since his powers all one thing this tiem. It could be a literal fusion where Ichigo can naturally use all the powers of shinigami and hollows/arrancar i.e. shikai, bankai, cero, resurreccÝon etc. ItÆs up to the writer though IÆm interested in the numerous possibilities.
Getting back to the plot, in the first scenario, everything that happens with Rukia is actually meant to be a set-up to trick Aizen into believing he had hidden the Hogyoku inside her. This undoubtedly screws up his plans when Aizen reveals himself when he discovers a) thereÆs nothing inside Rukia or B) a fake Hogyoku that might be a trap.
Aizen taking a fake Hogyoku to Hueco Mundo or being killed there and then could have a big impact on the upcoming events. In the latter case, Gin and Tousen are in the process of betrayal and thereÆs still a group of arrancar in Hueco Mundo. Though Stark wouldnÆt be one of the Espada and IÆm unsure which arrancars came about from the æperfectÆ Hogyoku.
In the second scenario, then Rukia really is the container and canon business goes down (or however the person writing the story sets it up). In this case, the most important factor of the idea is what the hell Ichigo is and how it might effect various plans.