Castle Wyvern is a dream world of mine. I loved Gargoyles. Whether it's Sora (or Riku) transformed into a Gargoyle (why not, it happened to Elisa) or simply getting a lift from Goliath or Brooklyn, it would be fun to soar above Manhattan, and recapture that KH1 Neverland feeling. I don't necessarily see it happening anytime soon, even with Demona in that mobile game. But it's not impossible either. I don't know if was popular at all in Japan. But I think Disney will cash on the nostalgia eventually, maybe let Jordan Peele do his movie adaptation, then we'll see.
Realistically, I can see Launchpad McQuack showing up. Maybe Magica De Spell too. I'd like for Donald and all the ducks to get a little sidestory. If they ever introduce Max into KH, and I hope they do, he should be an accomplished and independent knight (or something) in his own right, so it doesn't feel like Goofy's been neglecting him. He would be a cool party member too, and an excuse to bring back skateboarding.
I've never seen more than one episode of Phineas & Ferb or Gravity Falls, but they're both really popular, so I think characters from either of those could potentially cameo. Gravity Falls in particular has an occult theme and a nice Pacific northwest setting, so I'm sure it would work as a world too.
Kim Possible is definitely iconic, and has a unique 60's futurist style, but I'm not sure the setting is memorable enough to be a world. I'm sure Drakken had a cool lab, but I don't recall anything about it... Still, Kim, Ron and Rufus popping in on missions throughout the game, or just for one espionage segment in an original world, would be fun. (Although the Rescuers might be better for something like that.) I've always wanted them to put surprise Disney characters in the colosseum, should we ever see one again, and I feel like Shego, with or without Drakken, would be perfect for that. Demona too.
I think it would be funny if the Ashleys from Recess had their own secret exclusive accessory shop in Traverse Town, or a new original world with a creepy school theme. It's about time Sora had his education. Of course Spinelli would factor in, being the other Ashley. She's both the only character who could stand up to the Heartless and her name-based identity issues are kinda Kingdom Hearts, right? Randall could be Maleficent's little snitch.
Highly unlikely, but if we ever did see KH get a House of Mouse-level influx of random Disney characters mingling in original worlds, Rolie Polie Olie and fam relocating their teapot house to Traverse Town would be adorable.