Well from my point of view, Terra could be very well be Xehanort for the following points
1.) The trailer shows a scene where Master Xehanort and Terra talking about their personal darkness and Xehanort tells Terra to control it
2.) Terra is said to be falling into darkness by Maleficent
3.) Vanitas mentions in the trailer that Terra's existence is fading
4.) Master Xehanort said that who fall to darkness are unfit to to wield a keyblade
5.) Removing the X in Xehanort leads to a anagram "No Earth", which in conjunction to point 3 would make sense as no earth could imply that a earth that was once there but no more
6.) Terra's physical similarities to Xemnas and Ansem SoD
7.) Terra's blue eyes changing to yellow in the trailer
8.) (Unclarified) Xehanort finding and opening the door of Radiant Garden, which could be limited to potential, current and former keyblade warriors
So from these points, I can get that prior to searching Master Xehanort, Terra had darkness in his heart and Xehanort senses it and tells Terra to control it. Eventually, Terra follows Xehanort and while following him he is slowly succumbing to his darkness. Then during some point in time in the story, Vanitas and Ven fight and during the battle Vanitas tells Ven that Terra's existence is fading... Sooner or later, Aqua and Ven meet Maleficent, who tells them that Terra is falling into darkness... Then somewhere in the end, Terra falls to darkness completely and has a complete personality change due to it, including changing his eye color. He was defeated by Aqua, Ven and Vanitas , with his ability to wield a keyblade stripped, and left in Radiant Garden with Ansem the Wise's care. After that, we all know what happens afterwards....
Well this is just my personal assumption...