Honestly, that would be the least exciting choice for me, only because it doesn't have much visually to stand out from Dwarf Woodlands and Enchanted Dominion (but if we have a boss battle with Mad Madam Mim in that world then I ain't complaining). The Black Cauldron on the other hand would be a more interesting choice simply due to the fact that the movie is more obscure, it's visually different with it's darker colour pallete, and it has an infamous history with Disney. Both worlds seem more likely than my third option, and I would be alright if it was either of them, however...
The movie (well, short to be more precise) that I'm rooting for to be the world is The Prince and The Pauper. In my opinion, I feel as though that this movie would be such a perfect candidate. It's a Fairy Tale world right down to the premise, it's a Disney World/Disney Castle world for Kingdom Hearts Union X, there's a lot of creative liberties that Square Enix has because of its obscurity, it can be a world exclusive to Union X because of its obscurity, and there a lot of interesting ideas that can be played in this world if Pete did end up in the world (two Pete shenanigans).
As far as Enchanted Dominion goes, Maleficent did tell Terra that she has "Powers over dreams". What if Maleficent was aware of the circumstances of controlling dreams in the world that she was in and uses that to her advantage? That would be an interesting turn of events that demonstrates that Maleficent has the upper hand even against Xehanort? What's even more creepy is that Aurora falls into a deep sleep anyway so the parallels are bound to occur.
Pretty sure that Olympus Colosseum was in the game before Nomura made the statement of the Princess of Hearts worlds. When Nomura was taking about the worlds they wanted to choose for Kingdom Hearts X, he stated that they had to fit the Fairy Tale theme in order to qualify, which means, world's like The Grid (Tron Legacy) or Deep Space (Lilo & Stitch) would not qualify because neither are based off Fairy Tales, but worlds like Never Land (Peter Pan) and Pranksters Paradise (Pinocchio) would because they're directly based off Fairy Tales.