From the latest in the manga, I think the fullbringers must have helped Isshin get his powers back in some way. Thats probably why Ginjou was looking for him. And the way that everybody talks about Ichigo being "special" it could be possible that his mother was a fullbringer and if Isshin was a human soul reaper too, as we now know there have been others. Isshin may have offered to take Ichigo's mothers powers to help her. It might explain why ichigo is really powerful if he is part soul reaper part fullbringer.