I loathe the Terra=Xehanort theory with a burning passion, >.<.
I did to at first, but there are a lot of connections linking the two characters. Remember years ago, about 2 or 3, members as well as people visiting other kh sites, believed that Roxas was his own person, a young man living in Twilight Town, but we soon found out that he was truly Sora's nobody, which was a shock by some. Even though you hate the idea, you should at least take it into consideration, as far back thought or an undead theory.
It doesn't make any sense to me, since the way he talks makes it seem like it's AFTER the BBS trailer...
How so? Master Xehanort could have done something to Aqua and Ven, although they might have survived as he soon fines out towards his path to facing off against Master Xehanort.
That way, he can't be Xehanort, and those who think that Terra's not really there can't complain.
Well have you read the new development section of the theory? Koji came up with a thought and I kind of ran with it. Master Xehanort had an apprentice, who looked exactly like the Xehanort that Ansem the Wise taught. Due to the experiments, The young man was split into two beings. The light side of his heart, which was connected with half a vessel and half a soul, gave birth to the entity known as Terra, while the dark side of his heart, which was connected to the other half of the vessel and soul, gave birth to a new type of enemy, but due to his strong heart, he kept some what of a humanoid appearance. He became the DS. Terra eventually craved the darkness, while the DS craved the light.
Towards the ending of the Secret Endings, Terra fused with the DS, forming his original self. The young prodigy began to see what his Master was really was, perhaps through Terra's time as a being. The young man defeated Master Xehanort and stole his name. But something happened that caused Xehanort to lose his memories. I believe it had something to do with the activated Kingdom Hearts figure in the sky, maybe the power of the hearts got corrupt or something and Xehanort tried to absorb the hearts to slow down the destruction, but he probably over flowed with power and his memories may have left him due to such power, but anyway Xehanort's super human instincts could have came out of what happened to the heart shaped figure in the sky.
Sometime later, Ansem the Wise finds Xehanort in bad shape.
Terra in Final Mix+, I believe is present, the portal that Sora took was possibly something linking the to the past if you ask me. Perhaps Sora came across Terra after he believed his friends were defeated and before he finally took after Master Xehanort. We know that Terra was most likely talking about Master Xehanort.