Well he would have to be one of the charcaters now in the story or atleast be somehow linked...My guess is Xehanort
Yeah that's most people's guess, however i'm kinda thinking theres more to it than meets the eye. And the main thing that meets the eye is the hair. Terra's and Xehanort's....and Xemnas'. But yeah. XD I'm more for the whole DS=X theory only because well we haven't heard anything on the DS and he IS the one that spawned right from MX's body. What if DS+Terra=X? Dunno if anybody's thought of that. Course I dunno if its even possible. ;