I doubt most, if any, of these worlds will be in KH3 at all, but what's there to lose? Me list:
-Kaua'i from Lilo & Stitch. It's about due time that Stitch's story in the series becomes fulfilled. I like the idea of him just wondering around after BBS and suddenly finding Sora, but that should be elaborated a bit more.
-Treasure Planet. A longtime fav of many people and I consider myself one of those individuals. It could be the replacement of Pirates of the Caribbean for the next mainline game!
-Atlantis. I used to hate this movie but that was silly. Very interesting world and lore behind the fictional, ancient undersea city of Atlantis. I would definitely love to have the lore of this world tie into KH4's plot somehow (break the rules a bit, no world's story has to be set in stone).
-Destiny Islands. KH3 screwed us on this one. Not even playable.
-Twilight Town. Also kind of got screwed in KH3, but definitely had a better treatment than Destiny Islands. It was beautiful, but it wasn't big. I feel like the sewer was just added to somehow make the world seem a tiny bit bigger. The hole in the wall was fine. Just give us the old KH2 Twilight Town back!
-Radiant Garden. Wish it was in KH3, but it didn't have to be. It would be nice to somehow make this world have a strong return plot-wise. Plus after discovering different parts of the world in three different games (KH1, KH2, BBS), I really would have liked for most of that expansiveness to have a return.
-Neverland. I think that Neverland's treatment in BBS was pretty bare bones, but to each their own. I think with the new engine that this world could be pretty damn gorgeous and imaginative. The only downside is what story to go off of if not to develop an entirely original concept.
-Wonderland. There was a lot of content, characters and story cut out of KH1's Wonderland (and rightfully so considering the limitations). I think it might be due time to recycle those old and missed out ideas. The same thing goes for this world as for Neverland, as well, whereas with the new game engine and later gen console, new possibilities arise in terms of expansion.
-Andy's Room!!! Make Andy's house the setting of the Toy Story world this time. I know Pixar got involved in the story for KH3, but I think the world could have been more fulfilling if it took place somewhere more familiar. At least Pizza Planet would have been better, in my opinion.
Those are just a couple. I know I have some bland opinions on how KH3 handled their worlds, but regardless I really enjoyed the game.