Okay... here's how I can accept those two worlds being in KH3 (though obviously I still don't want to see them in):
A Final Fantasy world.
Radiant Gardens DOES NOT count. I mean an actual world based off of a Final Fantasy game. My vote goes towards something based off of FF13 or FF15.
A TWEWY world. (The World Ends With You)
This was actually hinted in the Traverse Town world in KH: DDD. I wouldn't be surprised to see this in KH3 regardless of Star Wars and Marvel worlds being in or not.
Either way, if those two worlds are in KH3, maybe just MAYBE I can accept a Star Wars and Marvel world.
That's pretty much the only way I could accept it though. Maybe.