In Dream Drop Distance, we had characters from The World Ends With You showing up in Kingdom Hearts. This has made me hopeful that we may see others.
While I have to admit that I kind of agree with the thread stating that the Final Fantasy characters are rather pointless, I still enjoy them, and their interactions with each other and with the Disney characters.
Seiken Densetsu, Chrono Trigger, Dragon Quest, The Bouncer, Fullmetal Alchemist (they hold all the game rights to the series, why not?), Radiata Stories, SaGa, Valkyrie Profile, Odin Sphere, Brave Fencer Musashi, The Last Remnant, Star Ocean, Vagrant Story, Lufia, Parasite Eve...
Heck, part of me really wants to see how Hitman would play out in the Kingdom Hearts universe, though I honestly think that it and Tomb Raider are not really going to be a possibility.
I'd really like to see a Bravely Default tie in. I can't say too much without getting into spoilers, but the game setup would be really easy to bring in to the Kingdom Hearts universe. Bravely Default Endgame Spoilers Ouroborous attempting to consume the Disney worlds would make for the most epic superboss the series has had.