I reckon that Orochimaru has some sort of different part in the whole plan, because it looked like the four hokage had kind of discussed their plan of action to stop the juubi. I do think that he'll show up very soon
And Sakura is a lot more powerful than she looks, it'll probably be some mega powerful chakra punch or something, maybe a new jutsu developed with Tsunade??
Orochimaru remained behind to help with the preparations for the "Welcome back!" party, along with karin =D oh, and suigetsu, too.
And sakura has a new power-up, maybe some ultimate jutsu tsunade taught her...