I still stand by my theory that Ven's soul somehow ends up in Sora somehow...which is how we get happy and excited!Sora, but then a Roxas appears when his soul is separated again.

That's assuming that Xehanort is loaded, which I guess he could be. |D I'm guessing that Terra still ties into Riku with earth/land sorta deal, and 'Xehanort' can be anagramed into a few interesting combinations...