Grass made a very appealing theory a few days ago; He mentioned that it was a 'new Xemnas'. That after Xemnas was defeated, he fully fused with his KH to become "more than human". However, since he lost, he didn't rise above human, but obtained his own heart and became the equivalent of a Somebody. Not Terra, but not Xemnas. "No Name"; a being with a KH of people's hearts (allowing him to access all the powers he has, considering it has their hearts) as his heart and Xemnas's body.
I don't remember why he said he'd fight TAV, but I can't recite it. The theory was very in-depth and long, but it calls out to what i think. That's my two cents