Princess of Mars
Disney studios will forever shudder at the mere mention of the recent box office flop John Carter, but the series' main character was nearly brought to the screen in an incredible fashion more than 70 whole years prior. Yes, Loony Tunes director Bob Clampett was set to create a fully animated version of one of the most popular Carter novels, Princess of Mars, and even created some very promising rotoscoped tests that recently surfaced.
What would have resulted surely would have been a classic. It was right at the very beginning of the development of full length animated films, and from the test footage, it could have given Disney a run for its money.