Nomura has stated a few times already that the Final Fantasy characters are just cameo appearances. Their world in the KH universe is Hollow Bastion/Radiant Gardens. (Unknown about Vivi and setzer however) Sephiroth and CLoud is just a sidestory of Cloud being angsty and battling his "Dark" side.
Nomura has clearly stated that Kingdom Hearts is the story of the Keyblade and Sora's involvement with it. While a FF fanatic like myself enjoy their presence, I do not believe he will or should add FF worlds.
Nomura has clearly stated that Kingdom Hearts is the story of the Keyblade and Sora's involvement with it. While a FF fanatic like myself enjoy their presence, I do not believe he will or should add FF worlds.