Mort

Mort was a planned traditionally animated film adaptation of the Disc world novel of the same name by Terry Pratchett. It was to be produced by Disney and directed by John Musker and Ron Clements, the director duo of The Little Mermaid and Aladdin.

Plot
Mort is a young man who Death whisks away to train as his apprentice. But on his first solo assignment, Mort prevents the assassination of a princess, threatening the flow of history and the fabric of the Discworld.

Why It Was Cancelled

 * 1) The rights to Discworld were sold as a package and Disney was unwilling to option the entire franchise.
 * 2) According to some sources, Disney higher-ups were hesitant about the idea of Death as a major character.

Result
Musker and Clements moved onto a new project based around Polynesian mythology, which would become Moana.