Here is a theory on Terra + Xehanort, similar to a theory I read on Youtube, but I improved it a lot.
To start off, I will tell you the theory straight. I think Terra IS Xehanort. Not the Old man in Birth by Sleep , but of Xehanort after.
The reason I think Terra is Xehanort is from appearance. The crazy hair, (back and down), the body build and the youth in general. But the main appearance trait that they both share is the Yellow Bright Eyes. You must be thinking, "Wait, Terra has blue eyes," this is true, but during the end of the Birth By Sleep cinematic from KH2 FM+, Terra is seen on the ground with his newly Yellow-tainted Eyes glaring at the Old man Xehanort------>
Now heres what I think happened after the cinematic... Terra,(with his newly Yellowed Eyes), is taking by the dark side because of his hatred of The Old Man Xehanort and the death of Ven(tus). He gets so angery and goes beserk, killing Old Man Xehanort and Aqua, (or Old Man Xehanort killed Aqua before then, but still got killed). He decides to change his name to Xehanort and to follow his steps to collect the 7 Hearts of Light,(Purity, etc.), Terra (or Xehanort), is completly evil...
Now back up, Why did he change from Good to Evil? Well, Melifecent (SP) qoutes that Terra is becoming evil, and many hints are given in the trailers to say he is evil. Terra also Ven's friend, which is a reason why he gets angry when Old Man Xehanort kills him.
Xemnas is also proof to say that he is Terra('s nobody), Xigbar has been hearing Xemnas talking in the Hall of Sleep(or awakening), and belief it or not, he is talking the the armor and keyblade of Aqua...
"My friend".
Hate or not to Hate, that is the question...