Historically, Kingdom Hearts games have been set in Disney worlds and only featured some Final Fantasy characters. It appears Kingdom Hearts 3 will be keeping that tradition.
During E3 2016, Gamespot had a chance to speak with Tai Yasue who is the co-director for Kingdom Hearts 3. From the sounds of things, only Disney worlds will be the main focus in the new game.
He said: “We’re not adding any Final Fantasy worlds. A lot of the original [to Kingdom Hearts] worlds [Hollow Bastion, Traverse Town, e.t.c.] had Final Fantasy characters in them. At this moment, we are not really coming up with any plans for that, we’re making original worlds and Disney worlds.”
The main reason they are choosing Disney worlds is because they are recognizable and famous. He also said Disney worlds are unique and they can implement a lot of gameplay ideas within them.
The other reason is because they want something different that adds a bit of variety. This is probably a way of saying that Final Fantasy worlds have featured in video games before. Some Disney films out there have yet to have fully featured video game worlds.