OK, my first theory wasnt much of a theory
But now I actually have one o/ .. Well, Ichigos spiritual power got away from him after Mugetsu right ? What if it didnt just dissapear but instead transfered to a strong human soul in the Karakura town - the new guy. Shirosaki did say "Farewell" which implies he was certain that he wouldnt die, only that he would part with Ichigo. The word "part" was even used in the translations !! Just remember Dragon Ball, Evil Buus power got transferd into some black kid. I think the same happend here. This would mean that Ichigo and the "new guy" would fight over Shirosaki and Ichigo regains his powers back again !