I'm also running with the theory that it is going to be a world that is probably not as heavily marketable.
I also have a feeling they are going to keep at least one world a secret. I don't think they have ever revealed every single world in the game ahead of release (I could be wrong about that, because I have a tendency to go dark before a game comes out and limit what news I see).
Either way, if we look at the worlds we know are in the game (I am guessing at the order):
Tier 1
1) Olympus
2) Traverse Town / Mysterious Tower
3) Toy Story
Tier 2
4) Radiant Garden/Winnie the Pooh
5) Rapunzel
6) Frozen
Tier 3
7) Monsters Inc.
8) Pirates
9) Big Hero 6
10) "Final World" that Nomura always wanted to do???
If we include original worlds as part of the "sets" of three, then by that logic there actually shouldn't be any more worlds to announce. But I believe there was at least a quote from someone saying that while they were working on the worlds in sets of 3, "that isn't to say there are only 9 worlds in the game..." I feel like that would be pretty lame to claim it is more than 9 and actually just mean 10. We also have no idea if WtP is considered its own world this time, or if it is going to be based in Twilight Town or not. If it is part of Twilight Town well that naturally frees up a whole space.
I have no idea how many they have yet to show, but I feel like it would be crazy not to have at LEAST one more world hidden. The problem is, I don't know if they are going to show us what it is before launch, especially if it is something not as marketable (like a return to Agrabah) or kind of niche (Atlantis) and/or not as well known in Japan (Princess and the Frog). They are definitely pushing the Disney franchises strongly, trying to use the big names of these movies to draw crowds. And like I said before, I dunno if they are actually going to show their whole hand before the game comes out.
So my prediction is that they will NOT reveal any more worlds, but that there will be more worlds in the game.
I also think there will be some smaller worlds that we will visit that might count as a "world" but won't be fully fleshed out. I think we will go back to Radiant Garden, but it will be fairly small in scope. I think we will go back to Destiny Islands, but it will be very small (as a stylistic choice this makes sense with the game's plot as well-- as Destiny Islands is supposed to feel "small"). I also think we will go to Castle Oblivion or Land of Departure at least, depending on the story. But there is no reason for those worlds to be massively expansive and that will probably rack up the "world count" list.
In terms of actual, large scale Disney worlds? At the MOST I think at least 2 more.
Like the others, I am putting my money on Atlantis and Princess and the Frog but would not be surprised by a reprise of Agrabah (please no) or something so insane none of us could have expected it.
After all, Nomura did say a while back that his favorite world had yet to be revealed, and that was after the Pirates announcement, right? So like...unless his favorite world is Winnie the Pooh...?
I'm wondering what world it could be.