Ven: "What do you mean that Terra will no longer be Terra?!
Vanitas: "I meant it exactly as I said it. The existence known as Terra will soon disappear."
My theory: Terra will no longer be Terra. Terra will become Xehanort. It will somehow be related to him giving into the darkness in his heart. Technically, Riku almost entirely became Xehanort, but he was able to rid himself of Xehanort. My guess is the same thing will happen to Terra, only this time he won't have the fortunate outcome that Riku had.
This does make sense and would fit the storyline.