I liked your idea of KH standing on its own two feet, really fascinating
The final fantasy characters were never too important though, mostly in
Traverse Town and Hallowed Bastion/ Radiant Garden
I saw the disney characters to have bigger roles in the story
In my opinion, they should just leave the final fantasy characters that are there but not add anymore. They could act as minor characters, only showing up in a flashback or something.
As for Disney characters, they should limit them as well(but keep the worlds! lol)
Id love to see more unique characters be made for the KH franchise. =)