Well, I've already posted my other theory a long while back, and now I am adding more to it. This is, however, my belief of what happens at the end of BBS, and how Terra comes to be Xehanort from KH2.
Now, we all remember that Ansem was able to take control of Riku's heart, particular when Riku's heart welcomed in the darkness. My theory is that this is what happens with Terra. After a grueling battle against Xehanort, Terra goes to finish him off. Both he and Xehanort are in a weakened state. Xehanort, knowing he is to lose this battle has one final resort: to put his heart into Terra. Even though Xehanort is not a heartless, it is possible that he has this power. However, both hearts are strong, and internally, Terra's heart is desperately fighting to keep Xehanort at bay, and Xehanort's heart is struggling to fight off Terra's. In the end, Xehanort's heart is the victor, resulting in physical changes to Terra's appearance (just as Riku's appearance changed when he used the power of darkness). But because Xehanort's heart struggled to win the fight, the chain links of his memory were scattered and lost, resulting in a new Xehanort with memory, only of his name.
Now in kingdom Hearts 2, we see that Xehanort has powers. His heart is strong, but no longer is he one of the keyblade's chosen ones. Now he is trying to piece together his memory, going to new lengths in experimenting with his own heart to do so, and even continuing that quest as Xemnas.
BTW, my apologies if this theory has been posted and challenged. Any thoughts, bashing, other comments are welcome.
Now, we all remember that Ansem was able to take control of Riku's heart, particular when Riku's heart welcomed in the darkness. My theory is that this is what happens with Terra. After a grueling battle against Xehanort, Terra goes to finish him off. Both he and Xehanort are in a weakened state. Xehanort, knowing he is to lose this battle has one final resort: to put his heart into Terra. Even though Xehanort is not a heartless, it is possible that he has this power. However, both hearts are strong, and internally, Terra's heart is desperately fighting to keep Xehanort at bay, and Xehanort's heart is struggling to fight off Terra's. In the end, Xehanort's heart is the victor, resulting in physical changes to Terra's appearance (just as Riku's appearance changed when he used the power of darkness). But because Xehanort's heart struggled to win the fight, the chain links of his memory were scattered and lost, resulting in a new Xehanort with memory, only of his name.
Now in kingdom Hearts 2, we see that Xehanort has powers. His heart is strong, but no longer is he one of the keyblade's chosen ones. Now he is trying to piece together his memory, going to new lengths in experimenting with his own heart to do so, and even continuing that quest as Xemnas.
BTW, my apologies if this theory has been posted and challenged. Any thoughts, bashing, other comments are welcome.