My theory is that Xehanort is actually Ven in Terra's body. In the trailer that's been released, Terra thanks Ven for saving his life, and it's also alluded to that something happens to Ven. In which case, I reckon whatever's happening to Ven, Terra tries to stop it by giving him his heart or something, thus causing Ven to become Terra and Terra (as Ven) to disappear- or go into Riku, seeing as Riku was the supposed to be the master of the Keyblade. When Riku accepted the darkness in KH1, that was then transferred to Sora which explains why Roxas looks like Ven? I dunno.
I'm also thinking about something to do with the Keyblade Riku had in KH1- the one made from the princesses' hearts. Can people become Keyblades? Or have their power stored in one?
Personally, I think it's too obvious that Terra = Xehanort. =/ My theory's probably wrong though. XD