Already considered that it was a Nomura game. But other big heads were in on it too. Also, Nomura's characters are generally the most famous FF characters, so it makes sense that they use them as mains, just like they mostly use the old Disney movies and not as much from the new ones or animated ones. KHII had 2 non-Nomura characters in 1 game and it's very likely there will be more, especially in KHIII, although Lightning probably will be the new major character there.
As usual, I am just stating possibilities. Not considering if there will a new SE character for every title at all. Merely saying that this opens a door. That TWEWY is a Nomura game does not change the fact that it is the first non-FF/Disney character ever to be in a KH. And if it has happened once, it can easily happen again. I can easily imagine Aya in there at some point, but probably with a title that has a slightly older cast, since it may yet be too mature for KH. Lol, they would include her for a special KH shower scene. Complete the game 358 times over 2 days to see it, and earn a rank of DDD.