Its probably going to be one of the biggest revelations of the game to find out the person behind the mask. Same as in the movies. I think we can expect some kind of dramatic history behind the bad guy/girl, maybe even mutual history with the PC. Although I doubt it will be Revan behind the mask.
What bothers me is why would Revan disapear after the DS ending. It does not make sence he becomes the sith lord with a huge army and the star forge to back him up.
I wander what they are going to say about the starforge, because if you do the DS ending it is not destroyed. And it is/was one of galaxies most powerfull weapons.