Ok here's what I'm thinking would be cool in Kingdom Hearts...
Ok as I've mention I think Darkwing Duck would be a neat addition to KH. The best thing for KH from DWD, would be the Fearsome Five (if you don't remember the show, that consists of Negaduck, Quakerjack, Megavolt, the Liquidater, and Bushroot) would join Maleficent's team as her new henchmen and therefore would also have some control over the Heartless. Which their would be a level in which you would take them on one at a time with the help of the Heartless. Than it ends for the Fearsome Five when Negaduck steals the powers of the other four, such as in the episode "Jail Bird." Now Sora, Donald and Goofy have to take on "Mega-Negaduck."
As for the Muppets, I was thinking a world based on the Muppet Movie, in which Sora, Donald and Goofy met Kermit and Fozzie whom are on there way to Hollywood. Along the way they run into Doc Hopper, whom like in the movie wants Kermit to be the mascot of his chain of froglegs restraunts. Of course the frog says no. Over the course of the level you can have Kermit join your party, and also have to deal with the frog killer Doc hires. But in addition to the frog killer, Hopper was also able to get the services of the Heartless, whom was there due to the greed for money and power of Hopper's heart and the fact that he cares nothing for Kermit having morals. After the world is done, as a thank you of sorts, Kermit has another Muppet, maybe Sweetums, Animal or perhaps Gonzo, help Sora and gang, as a summon character.
I don't know, that's just what I was thinking would be neat. I'm not saying it would happen, I'm just saying that's my vision,