I'm pretty sure that KH3 will have the whole main cast. I mean, just look at 3D's ending. It made it kinda obvious that some people are going to have a bigger role. And I think that will include Kairi.
Spoiler:- Spoiler KH3D:
KH3 should also be multiplatform. There's no reason for it to not be, now that all mainstream home consoles are HD. Only minor differences will be necessary, and it will be more accessible for ALL it's fanbase, so why not?