Here's my guesses for worlds in KH4 (or whatever they title the next one):
- Scala ad Caelum/Daybreak Town. Either of those seem likely as an original world. And it plays such a big part in UX/KH3 stuff as well that both make sense. Especially since they are one and the same.
- Sugar Rush/The Internet (Wreck-it Ralph). Vanellope would make sense as one of the New Seven Hearts. Ralph already was a link summon in 3, so it'd be a "reunion" world for Sora. Sugar Rush racing minigame.
- Windswept Isles (Moana). This would continue the trend of newer Disney films being the focus, Moana herself could be one of the New Seven Hearts. The heart of Tafiti would of course make sense in KH terms. Question is, would open sea travel be the case (as in The Caribbean), or would it have more of an island focus and just hop between?
- Arendelle (Frozen 2). This could benefit from the melted Arendelle (including the main town) and whatever the sequel plot will be, or just make an entirely original plot. And of course we have Anna and Elsa both being New Seven Hearts. Only one of a couple returning worlds from 3. Think KH2 style.
- Zootopia (Zootopia). This would be a world that could finally give an excuse to make Sora his lion-ish self from the old concept arts. A bustling environment with many scenery changes. And personally one of my favorite movies.
- Toy Box (Toy Story). Also a returning world. This world was a great one in KH3 and I'd love to see what else they could do with new areas (the carnival from Toy Story 4 perhaps?). Also, Bullseye, Jesse, Bo Peep and Slinky please.
- Shibuya (The World Ends With You)]. Having a Square Enix hub would be nice, and we already had Sora and Riku meet Neku and the gang. Also, the secret ending.
- Agrabah (Aladdin and the King of Thieves). As much as everybody seems to hate this world, I feel like one last visit wouldn't be bad. Based on the third movie and with Unreal Engine 4, they could have the palace and the turtle island as big sprawling areas. Any non-Jafar villain. More info on how the old princesses connect to the New Seven Hearts from Jasmine, and some closure. Then we can abandon this world along with Olympus lol.
There's other possibilities as well but I'll have to think on it a while. As much as I'd LOVE Treasure Planet, Lilo and Stitch, and Atlantis, I somehow think we'll never go back to older films anymore. Can't shake that unease.