Love how this entire discussion side-tracked from the thought of Xehanort shape-shifting into Ansem SoD and Xemnas, to theorizing being a complete, and utter waste of time. Anyhow...
Spoiler ShowI concur with Grass's theory, a shape-shifting Xehanort, I called that once, and it was Xehanort and Terra constantly shape-shifting, fighting for domain over the heart.