I thought it would be interesting to post what I believe that would make cool new worlds to the Kingdom Hearts saga. Hera are my picks and why:
Come on,who wouldn't want to fight alongside Goliath? It has a dark tone with a little of disney on it. The world could have a medieval side and New York city, cool gargoyles-shaped heartless and a ton of monsters to fight. I don't know if this world would have a princess but still.
A Marvel universe world:
Yes please! How awesome it would be to play in any part of the Marvel universe, play next to the avengers and be able to "summon" The Hulk. With Guardians of the Galaxy on its way this world can go cosmic. The princess ... no princess haha.
Not so far away from possible! Think about a lightsaber keyblade, fighting alonside Obi Wan, Luke, heck even R2 D2! It would also be a good opportunity to do a couple of missions on the gummi ship, take down AT ST's, Star Destoyers, the Death Star! The princess of course ... Princess Leia.
A Final Fantasy inspired world:
If characters from the Square-Enix universe have come to the Disney universe why can't it be the other way around?. A world inspired by places like Midgar, Gold Saucer, Alexandria, Zanarkand, you name it! Fight alonside a Moogle, Cloud, Zell, Vivi, Zidane. The princess of this world, I don't know could be Aerith, Garnet, Rinoa, Yuna ...