Moral judgments aside, Leni Riefenstahl was an extraordinary visual stylist, which is why it's a shame that she was never able to realize her proposed adaptation of Heinrich von Kleist's play about the Amazon Queen Penthesilea. By all accounts it would have been a lavish epic shot on location in Libya, but then a little thing called World War II happened and it had to be scrapped.
Technically, this could still happen but it looks more unlikely with each passing year. At various points it has been set to star John Belushi, John Candy, Chris Farley and, most recently, Will Farrell. In fact, the Will Farrell version was pretty much entirely cast but then financing fell through and the project seems to have been dead ever since.
A musical based on the life of William Shakespeare. To be written by Anthony Burgess (of A Clockwork Orange fame, amongst others). To be directed by Joseph L. Mankiewicz, the writer/director of All About Eve. This either would have been brilliant or the perfect storm of WTF?
Frankly, this whole list could have been made up of Orson Welles' projects that never got off the ground, but this one is particularly intriguing. Just imagine if you could enjoy a double feature of Welles' take on the Joseph Conrad classic and Apocalypse Now.
Kubrick was (arguably) at the height of his genius when he was making plans for this movie - between 2001 and A Clockwork Orange - but the scope of the project (Kubrick reportedly read 500 books on Napoleon and planned to follow him from birth to death) ultimately resulted in the plug being pulled on it.