Hmmm I dont really know. The morphing idea seems a little vague to me too. So I was thinking that I read somewhere 'master xehanort's avenger, his name is terra' so that kinda made me think that xehanort is just plainly terra... but terra thats been through enough to not remember how he used to be.
I came to that by saying terra was a very powerful apprentice, and so MX wanted him as his apprentice because whatever MX's goal is terra (or any of TAV but I'll say terra coz it looks like hes the one that shit happens) to would help him somehow. He doesn't want DS for whatever it is, DS has allowed himself to use the darkness and is therefore unreliable. He says it to Ven, when he asks ven to join him coz DS is to dark. Ven denies obviously. But terra eventually gets too close to the darkness to refuse whatever MX is offering him, power, maybe. So at the end of the trailer aqua sees ven isn't quite dead yet, but their fighting a losing battle so its the ideal opportunity to erase Vens B+S and let his heart fly away to Ven trusts to be safe enough until something happens and he can regain his body. Aqua... gets killed. DS... gets killed? Im confusing myself now.
Anyhows, ill finish it for the hell of it. So MX and T are partners in crime right? wrong. Terra was only close to the darkness- he still misses his friends. and he knows one day, if vens heart is safe, ven will be able to return. so he gives riku his keyblade. Mickey and TAV's Master try to stop MX, and they do- but TAVs MAster has to do some suicide attack to do it. It kills MX and makes all of terras memories fall dormant- to bad he only remembers his enemys name.
Nah thats stupid. tell me what you think though, didn't write it for nothin'