[Bassoonicmayhem]

I think Isshin could be a Shiba, but not as a reincarnation or anything like that. I think that he possibly was Kaien's and the gangs, uncle (which would inspire Kaien to become a Shinigami) and then one day he went to the human world, probably saved Misaki and fell in love. Or he was already in love with her and something happened to her, idk, mortally wounded and then Urahara pops out of nowhere and tells Isshin he can save her if he gives up his powers (something similar to the Rukia/Ichigo thing... kind of... not really). He does it. Then acts like a normal human, or as normal as Isshin can get, and Misaki falls in love with him. Then they have Ichigo and you know the rest of the story from there.

The only problem with that is if Isshin gave up his Shinigami powers it would have been very hard from him to get them back without going to the soul society like Rukia did. (This could explain why it took 20 years for him to get them back though.) So, maybe he did have them locked up because he didn't want to go back to the soul society or hurt Misaki (something like that), but the way he acted around Urahara makes me think that Urahara did something that caused Isshin to lose his powers. But that's my crack-pot theory. I know it's kind of cliché and lame, but it wouldn’t surprise me if that happened.