I jumped into this thread thinking "No! NO! A THOUSAND times NO!!!". Disney would never let it happen. They're obscenely overprotective of their IP. Second, D&D still hasn't completely shaken its' bad rap and D20 is still too closely linked to D&D for them to go for it, I thought.
But, having read through some of the responses, it might just happen. There IS that Kingdom Hearts game, and if any hobby rivals D&D for its' "debasing-the-minds-of-our-nation's-youth" reputation, it's video games.
Also, a lot of the properties mentioned would make really good settings. Disney's Gargoyles cartoon could make for a really kick-ass d20 Modern setting, I think. And how about a d20 Gummi Bears? I even wrote up a 2e PC race for Disney's Gummi Bears, as a joke.
I don't think it's likely to happen, but I think it would definitely be viable, especially for the younger crowd.