2. Kingdom Hearts
It's been 11 years since Disney and Final Fantasy fans were first enchanted by Kingdom Hearts. The March 2002 release from Square Enix combines the two franchises as seamlessly as possible by having the main character, a boy named Sora, travel to different worlds in search of his friends, and in search of keyholes that he must seal in order to protect the worlds from the main antagonistic force, the Heartless. The game, and resulting series, has won several awards and has been outfitted for multiple platforms.
Why it would make a terrible film: A film could never mesh these franchises so perfectly. A live action film would mostly be out of the question, and a high end CG movie similar to Final Fantasy: Advent Children would likely have to have an enormous budget, both for paying key voice actors and for creating quality CG characters out of the animated Disney ones. In addition, the movie would be either horrifically long or grossly gutted due to the length of the core plot. Now, an animated series of Kingdom Hearts on the Disney Channel would be both lighthearted and appropriate given the established plot. More adults out there would watch it than they care to admit, you can bet money on that.