Ok let me see, so we know the game will tell us the past of Xehanort.
Since that person appears only at the very end of the game, it's safe (and by safe, I mean obvious) to say he is one of the main characters.
So, that leaves us with only those people :
Master Xehanort
Ven
Vanitas
Aqua
Mickey
Terra
Master Eraqus
So it's certain it's not Ven Aqua Mickey and Eraqus.
Vanitas ? Way unlikely, he's way too small, he absolutely doesn't look like him from what we've seen, the voice doesn't match at all neither...
So the only possible choice is MX or Terra. They both have a lot of connexions to Ansem, Riku, and Xemnas.
And well, I think it would be utterly lame to see Terra turn evil at the end of the game(confirmed by Nomura that was the climax of the game) and then just die 5 minutes after.
If it was MX, we could say he somehow absorb Terra, thus getting young and look like him... But it could very possibly be the opposite, Terra absorbing him in order to gain more power (we've seen repliku absorb Zexion's darkness, so the process is possible) might result in him being, well, like Xehanort. Then I guess Aqua will fight him and he'll lose his memory somehow.