I think that the only worlds that need to return for gameplay purposes are Pride Lands (it already used big areas, and they would want another transformation for the protagonists) and a mini game one. However, it would be weird if they were building up to Stitch finally returning to Hawai with BBS and KH2 and then it never appeared in KH3, but since his movies are not that popular anymore, I would understand that.
A for new worlds, I think there will be only two more, Frozen and another one (I hope it is based in a classic 2D movie if it's not Star Wars). Taking into account that we know Land of Departure or whatever is called right now is returning, we have 10 worlds and that's not including Keyblade Graveyard (I don't want it back and would rather have a brand new world for the finale) and Radiant Garden.
1. Twilight Town and the Mysterious Tower
2. Tangled
3. Big Hero 6
4. Toy Story
5. Frozen
6. Star Wars/Anything else
7. The Lion King
8. Mini game World
9. Hercules
10. Radiant Garden
11. Keyblade Graveyard
12. Land of Departure
That's 12 worlds and even with that number I feel like the only Disney worlds left to reveal are Frozen and another one, since the scope of the game seems too big with 13 worlds. As for Destiny Islands, Daybreak Town and Dark World, I could see them being relegated into cutscenes only, if the later is not playable being Riku-only. I don't think we should set up ourselves to be disappointed, I'm already expecting another XV moment in which the fans pray that there is a lot other continents that they haven't shown up to release... only to discover that everything that was shown almost a year before is what there is in the game.