The great comic novel A Confederacy of Dunces seems tailor-made for a movie. But the first three actors lined up for the lead role were John Belushi, John Candy, and Chris Farley—who are all dead. Will Ferrell now may be attached. Please don't die, Will Ferrell.
Mental Floss has more crazy tales of unfinished movies here:
[H/T: Mental Floss]