Disney had a copyright expert on set to ensure no infringement occurred. The production team worked under the constraint of abiding by the stipulations set forth by Warner Bros., the legal owner of the rights to iconic elements of the 1939 film, including the ruby slippers worn by Judy Garland. Therefore, Disney was unable to use them nor any original character likenesses from the 1939 film. This extended to the green of the Wicked Witch's skin for which Disney used what its legal department considered a sufficiently different shade dubbed "Theostein".
Disney had been struggling to produce an animated film based on Don Quixote since the 1940's.
If you go to the second link, there are a bunch of unproduced animated films based on novels, short stories, fables, legends, myths, fairy tales, etc. listed there.