Well this is just a guess but it fits.
Ever since i found out about the whole oue ken(key) and the that king of soul society relinquished his power to chamber 46 or whatever it was a lot of things seemed to click.
This would explain why ichigo and his dad are so powerful and it fits with ichigo's dad's personality. Also explains why uruhara would know him. As in only captains would know the king, and uruhara was one before.
The only hole that i can see is that no one seems to react to the name kurosaki when they hear it in soul society, guess his dad could have changed his name obviously but i dunno.
Well that and that if Aizen's goal is to assassinate the king of soul society and it is ichigo's dad he wouldn't need to go to that special zone.
But i still think that this is whats happening i mean Aizen might not know that the king left so whatever and if this isn't whats going on then they might be part of the last royal family but yea like i said i thought it fit.