This would be totally awesome! I was wondering why the producers never got permission from Universal to use LBT since the franchise has always been considered a rival to Disney. If this were to exist, this is what I see:
1. Sora would be a longneck, Donald would be a flyer, and Goofy would be a spiketail. For the sake of Jiminy, I see him as a swimmer.
2. Although it would be kind of hard to imagine, I would have the three of them fight with Littlefoot and the gang.
3. I think creating a new story using an existing enemy would work fine. I think this would be perfect for the heartless to recruit all of the major enemies of the gang such as Ozzy/Strut, Rinkus/Sierra, Dil/Icky, some sharptooth, etc...
This makes me remember the fanfic A Time Before Twilight, if I got the name right, by KingdomKey, again if I got the name right. He had the three of them in those respective forms and the gang were fighting the heartless.
I say this should happen!