Thinking of the people who "must be freed from their torment" (Terra, Ventus, Aqua, Roxas, Axel, Xion, and Namine), we know the current location of at least 4 of them is within Sora and Riku. These would be Terra inside Riku via Xehanort's Heartless and Roxas, Xion, and Ventus inside Sora. In other words, more than half of the tormented people.
My theory is that the data Ansem the Wise put inside Sora was developed as a way for these people to leave their host bodies without actually damaging the host. Ansem the Wise was fiddling around with the data forms of hearts, right? We also know from Riku stealing Roxas's money in KH2 that data-based objects can take a physical form, so a data-based body emerging in the real world wouldn't be out of the question. Perhaps Sora and Riku will delve into themselves and search for Roxas, Xion, Ventus, and Terra, pulling them out into the real world with data-supplemented bodies/hearts in the end. Kairi may end up being a playable character, too, because another tormented character, Namine, dwells inside of her. Then again, Namine has said that she "can come back" because of the promise she made with Sora at the end of CoM. But! I digress.
While inside his own self, Sora could feasibly run into any enemy that he, Roxas, Xion, or Ventus had faced in the past. Riku would probably encounter enemies from his and Terra's past as well, like the Riku Replica and Braig, but would also encounter loads of different Xehanort incarnations. Xehanort's Heartless (Ansem SoD) would obviously be a recurring foe, but we might see Xemnas, Master Xehanort, and Apprentice Xehanort (Terranort). Vanitas could easily show up in either instance, given his apparent connections to both Ventus and Riku. A Kairi storyline seems as though it would be rather...easy, though, given that she's a Princess of Heart and hasn't really engaged in battle. However, Namine has been through quite a lot, even if she hasn't done much fighting.
An even cooler idea is...maybe as you rescue each of the other characters, you are allowed to play through the game as them, or switch between different characters. I mean, I doubt that Roxas would just sit back and let Sora do all the work when he's trying to save Xion, and Ventus wouldn't sit still either if he thought he could help save Terra.
After this game is over and done with, Sora, Riku, Roxas, Xion, Terra, Ventus, and possibly Kairi and Namine will begin their journey to the Realm of Darkness to save Aqua, Ansem the Wise, and Axel. Why would Axel be there? I think Nomura once mentioned something about how a Nobody and Heartless must be defeated in the correct order, or something like that. Nomura mentioned something about a new character, too, so perhaps there is another hero who has been trapped in the Realm of Darkness since the Keyblade Wars? Time apparently flows differently in the Realm of Darkness, as evidenced by Aqua and Ansem the Wise's meeting at the end of BbS. This game would be the last one before KH3.
The only issue with this I can come up with is Nomura's mentioning of "new worlds," which kind of throws things off.