Ok. So we have established the fact that Sora will be trying to save Terra, Aqua, and Ventus. Right now I would say that Aqua is the easiest to say. All that is needed is to find a way into the realm of Darkness. For Ventus a.k.a Ven it is a matter of restoring his heart and then Sora giving it back to him. Terra is where it is tricky. Supposedly both Terra's Heart and Body have been consumed by darkness. His soul I believe still lingers in the Lingering Sentiment. I believe that both Terra's Heart and Body are in the Realm of Darkness and can be brought together. I believe this could also be true for other people say for example Axel. Of course this theory has some holes because MX's heart was in Terra's body and supposedly they are still fighting for control. It's quite possible that you could end up fighting Xehanort in his true body in the next game if the same were to apply to him as to Terra.