Seriously. If we count the Realm of Darkness as a "world", we don't technically need any Disney worlds in this game.
There are almost always 13 worlds in KH games, either all playable or including cut-scene exclusive worlds.

 

We already have that many original worlds.
Destiny Islands
Disney Castle/Timeless River/Disney Town
Mirage Arena
Radiant Garden/Hollow Bastion
Keyblade Graveyard

End of the World
Traverse Town
Castle Oblivion/Land of Departure
Twilight Town
Mysterious Tower
The World That Never Was
Realm of Darkness

I know what you're thinking "That's only twelve. Fail." But there is another
Kingdom Hearts itself.

We've already been there at least once, in the final battles of KH III. in all likely hood we'll see wven more of it in KH III. Possibly even be able to explore it. 

So there you have it there's no need for Disny worlds in KH III. It would be boring as hell without them and I'd hate that to be the case, but they aren't absolutely necessary for the usual 13 world limit.

Disclamer: I DO NOT WANT A KH GAME WITHOUT DISNEY. I AM MERELY STATING THAT IT IS POSSIBLE TO CREATE A FULL LENGTH Kingdom Hearts GAME USING ONLY THE WORLD SPECIFICALLY CREATED FOR THE SERIES, WITH NO ORIGIN IN THE DISNEY CANON.

Edited September 10, 201312 yr by Isamu_Kuno

LIES!!!!!

We need the Disney element, or KH becomes a FF rip-off.

but kingdom hearts is technically the final fantasy of another reality (according to jimminys journal) it was make by the creators of final fantasy and and has been created from the final fantasy director tetsuya nomura so....yeah take that buddy

The Disney element helps embellish the main themes and elements of Kingdom Hearts. I would argue it's extremely necessary. 

 

For example: consider how nearly every Disney world in Kingdom Hearts II involves some kind of dual personality, or faux personas, or questions what it means to be genuine and unique, or what it means to be a person. The Disney element helps push forward the questions posed by Roxas's existence, and the idea of the nobodies and existing.