How much closer could I be?
If you could add an anime world in KH3, which of course won't happen, but imagining doesn't hurt, which would it be?
Let your imagination flow! Let go of "this doesn't fit the series" or "that will never happen", just imagine ~~
I'd say an Attack on Titan world would be amazing in terms of environment. The player could use some kind of Flowmotion to just fly around there, attacking Heartless and Titans in a totally different, epic way.
My second option: a... Kill la Kill world! Not because of the nakedness (even though that would be something if you think about the right characters :> ) but because of the EPIC opponents the player could face. Think about fighting Satsuki alongside Ryuko. Using a cool Reaction hinting at scissors cutting Life Fibers. And the music, ohh, that rock music.