I don't know about Multiplayer

I don't know about Final Fantasy worlds. Though I'm sure everyone's still holding their breath over Vincent Valentine making an individual cameo. Personally, I'd like to see Genesis. Maybe it's because I like red.

Your idea for the final battle sounds interesting.

If drive forms returned, I think I want them to have default Keyblades. Shooting Star for Wisdom/Sage, Kingdom Key and Kingdom Key D (Mickey's) for Valor/Frenzy, Guardian Soul for Master/Adept (I always felt it was lame that three forms could dual-wield), Oathkeeper and Oblivion for Final/Flawless. Another option could be that Master/Adept could tri-wield and Final/Flawless could "storm"-wield (my way of saying four keyblades).

I don't know about the ending either. Xion and Roxas individualized from Sora (you didn't even bother with Naminé)..? I suppose Xion stands a better chance of being revived as she was a doll which means her body was not merely a physical manifestation of a part of Sora. But for Roxas to exist as an individual, either Sora or Ventus would have to be an incomplete existence. Basically, Sora would get this kind of happy ending only in his dreams.