The only worlds not yet confirmed to return are End of the World, Neverland, Wonderland, Monstro, Traverse Town, and Deep Jungle. Deep Jungle absolutely cannot return due to licensing issues, Traverse Town would have little point since all the characters in it returned to their homes (it was a sanctuary for those whose worlds were destroyed): the disney characters have gone to Disney Castle and the FF characters to Hollow Bastion and elsewhere; finally, End of the World is unlikely to return due to the worlds no longer being destroyed (it was the pile of world remains, not Kingdom Hearts itself). However, it might return in a fashion to surround the World of Twilight if the Nobodies destroy enough, or the World of Darkness if the Heartless do again. That leaves just Neverland, Monstro, and Wonderland to return as worlds. They already have a pirate world, so Neverland is extremely unlikely (according to Nomuras way of choosing worlds), and so only Wonderland and Monstro are left.
How would they put Wonderland and Monstro in KH2?
However, there is still the one new Disney World that Buena Vista promised us (and the rest of their prophecy has come true).
So, in the end, we can have one more original world, and possibly Wonderland and/or Monstro. Wonderland is likely, since, if the Princess' are getting stolen again, Alice would need to be there. However, thought the level did not follow the movie as well as it could, it was still mostly there. And following this train of thought, unless we get to go to Pleasure Island, Pinocchio has served his purpose in revealing Riku's plan in KH1.
For this original world, precedence indicates that it would be tied to one of the movies with Disney's "Classic Villians".
These are: Shere Kahn, Shan Yu, the Notre Dame guy, Cruella de Vil, the Horned King, Yzma, Scar, the Evil Queen, the Wicked Stepmother, Hades, Hook, Chernabog, The Queen of Hearts, Ratigan (I think this is the mouse detective villian's name), Ratcliffe, and the Rescuers guy.
The unused ones are: Shere Kahn, the Notre Dame guy, Cruella de Vil, the Horned King, Yzma, the Evil Queen, the Wicked Stepmother, Ratigan, Ratcliffe, and the Rescuers guy.
The Jungle Book would be unlikely given that Lion King is already present; however, Cruella De Vil, the Evil Queen, and the Wicked Stepmother already had characters from their movies in the game. This may either throw them out or indicate them.
If it doesn't indicate them, then Notre Dame, Horned King, Yzma, Ratigan, Ratcliffe, and Rescuers are left.
Notre Dame does not seem to have much of a supernatural element to it, and the priest, while controlling, would not ally himself with the Heartless or Nobodies. He is a purist, not so much powerhungry. Ratigan and the Rescuers movie would have almost the same theme, and would both require a less supernatural, more just odd setting. We'll throw them out.
Now we have Yzma, Ratcliffe, and the Horned King. A Pocohantas world would basically be a repeat of Deep Jungle, so lets get rid of it.
We have: Yzma vs the Horned King. Both worlds are supernatural and have a villian that would easily resort to evil and darkness to get their wishes. However, the Horned King would be almost too evil to cooperate with the other villians and their worlds' somewhat bubbly attitude. I'm going for Yzma's world on this, though popular demand may easily put the Horned King back on the throne.
In summary: We've got Wonderland and Monstro as distant possibilities, and the Emperor's New Groove vs. The Black Cauldron. I'm voting for the Emperor's New Groove being the next (and final, unless you count Deep Dive or Kingdom Hearts) world to be revealed.