I think Master Xehanort is trying to break in Ansem the Wise's lab to do experiments with the darkness in people's heart. Since Terra is infatuated with darkness (he comes across Maleficent in some pictures), it would probably be easier for him to some how summon Kingdom Hearts and fuse with Terra and become an apprentice. From there, he is now known as Xehanort, rather than Master Xehanort because he's not a keyblade wielder anymore. He forgets his memories, and then is discovered by Ansem the Wise. Once Xehanort disobeys Ansem the Wise's rules and conducts experiments with the darkness in peoples hearts, he becomes Ansem and Xemnas. Now Terra and Master Xehanort is split once again, but in a different form. Master Xehanort is more of Ansem (hence the similar clothing), and Terra is more of Xemnas (hence the remembrance of Aqua). Terra said something will happen to his body when he meets up with Master Xehanort and Braig Terra said the he may need Ven to save him.
Now think back to a few trailers and here are some interesting things..
Terra tells Ven that he was saved by him once MX does something to Ven's (heart?) MX tells Ven to give something up
Now if you look at this it starts to fit together.
Something DOES happen to Terra's body and Ven saves him somehow. MX then focuses on getting Ven out of the picture which ends up with that scene where Terra asks what happened to his heart. MX will try again to do something to Terra as to what we don't know, and without Ven this time it might work. +++++++++++++++++++
No just to add in a bit of theory...
It's possible that whatever happens to Terra is because of Ven falling into a trap and getting knocked out of the picture so he was unable to save Terra a 2nd time. Skipping a head I think it's possible that Destiny island scene with Ven and what looks to be Aqua might be the end of the game.
Because of Ven falling into a trap the existence of Terra is no longer Terra. In the end Ven tells aqua to erase him, because he feels guilty and wants to have his existence erased as well. Aqua does something to erase Ven and some part of ven becomes one with sora.
If you think about it this would be very appropriate. Because that could make it like this, the existance of Ven is no more and in his place is the non-existent Roxas, and the existence of Terra is no more and in his place is the non-existant Xemnas.
Personally with all the KH symbolism I could really see that possible.
Anyway yeah just was kinda thinking and seeing what I could piece together.
