I would love a Kingdom Hearts fighting game, if it was in the vein of Dissidia I would love that. Seeing Sora, Donald, Goofy, Riku, Mickey, Pete, Minnie, Mulan, Shan-Yu, Ariel, Ursula, and all the different Disney party members and their villains from the films show up.
The Princesses of Heart could even participate as Assist only characters, though Kairi would be playable due to her beginning her keyblade combat training at the end of KH 3D.
The Princesses of Heart could represent the 7 Lights, and Organization XIII could represent the 13 Darkness's. Which could eb the KH versions of Dissidia FFs Cosmos and Chaos.
and of course Maleficent would be like Shinryu and Cid, the one who orchestrated it all.
But yea, I would love to play as Mulan, fighting along side Aladdin, to take down Ursula and Oogie Boogie. A dream to me really, would be epic!
and the FF characters ... eh, I guess they could re-cycle Cloud, Tifa, Sephs and Squalls Dissidia FF attacks in a KH Dissidia. But keep them as bonus characters, after all the SE characters have little, to almost no, importance in the KH Games, its all about the Disney characters.