Hi. Well the author is not yet making the "BACK-UP" story for that. It's better to wait, i guess..

i must say that if the last option was true ... i. i don't know what i would do Oo. But it would be quite strange xDD

IMO he's a shinigami who fell in love w/ a human woman and chose to spend his days on earth in a gigai body instead of living it large in Soul Society and by the looks of it he seems to be one powerful shinigami at that.
Another story that could be is that he fell in love w/ a semi-Arrancar Hollow which was Ichigo's mother and hence why he has this double shinigami-hollow persona, but this is unlikely.

Your theory is kinda you know believable coz how the hell does ichigo has so much reiatsu i think he has an equal or greater tha aizen

Ok....well it's true that he did fall in love with a human, I know that for sure. But, I've been seeing that he's been in 0th squad (a.k.a. the royal guard). Plus he has the captain uniform...

I think it might be something different. I think Ichigo's father is actually part of squad 0 and that the reason he is in Karakura town is to prevent anyone from making the key required to get to the royal family. It would kinda explain why his powers were locked until recently because squad 0 is only responsible for protecting the royal family and thus they would not be allowed to use their powers in the human world until a threat to the royals arose (in this case aizen). It would also explain his and subsequently Ichigo's massive power. He also seems to have a lot of knowledge about things that regular captains don't seem to know.

Either that or he left squad 0 and the person that Urahara replaced as captain was promoted to take his place. In either case I doubt that he is a regular captain.

But this brings up the question of how did he meet Ichigo's mother? Because captains don't normally enter the human world. So far they have only come out in exceptional circumstances (runaway shinigami, bounto, and aizen). In fact it has yet to be mentioned that there were any major problems with policing the human world till Ichigo came along. For his dad to have spent a long period of time in the human world with Ichigo's mother as a captain is pretty unlikely.

The other option is that his mom's soul was in soul society but got put back into a body and thus his dad followed her to earth, but it still wouldn't work because she'd be a baby and thats just weird........also for all intents and purposes when a soul is reintroduced to a body their former personality gets erased. I just don't see it being likely that his dad is a regular captain. The circumstances required for events to play out as they have are too far fetched. Then again this is manga so it probably will turn out like that.

I think he is most likely the former tenth squad captain cause thats the only squad that hasnt shown a single former captain before toshiro.