There's a terrific list of unproduced and unfinished film projects over at Film Comment's site.
Here's a few below that would have been tantalazing.
The Autumn of the Patriarch (Emir Kusturica)
Kusturica discussed an adaptation of this novel with author Gabriel García Márquez. Marlon Brando was to star, with Sean Penn producing.
The Demon (Sergei Parajanov)
Based on the long 17th-century poem “Demon” by Mikhail Lermontov. The once-banned poem praises the eternal spirit of atheism.
Genesis (Robert Bresson, 1963)
A lavish adaptation of the Book of Genesis. Dino De Laurentiis had agreed to finance, but Bresson abandoned the project only to take it up again and then abandon it a second time. He once said that one of the frustrations with the production was that he couldn’t make his animal performers do as they were told.
Kaleidoscope (Alfred Hitchcock, 1964-67)
After watching Antonioni’s Blow-Up, Hitchcock felt he was a century behind the Italians in technique. He asked the novelist Howard Fast to sketch a treatment about a gay, deformed serial killer. Pleased with the results, Hitchcock composed a shot list with over 450 camera positions and shot an hour’s worth of experimental color tests. MCA/Universal were disgusted by the script and immediately canceled the project, reducing Hitchcock to tears.
The Monster Maker (Alain Resnais, 1970s)
In collaboration with Stan Lee, Resnais planned a pop-art parody about a frustrated movie producer who seeks creative and spiritual redemption by making a film about pollution. Lee and Resnais sold the script in 1971 but it was never made.
[Read that again!]
The Streets of Laredo (Peter Bogdanovich)
Co-written with Larry McMurtry, this was to star James Stewart, John Wayne, and Henry Fonda. A decade later, McMurtry turned the screenplay into a Pulitzer Prize–winning novel. John Ford talked Wayne out of doing the project.
Suffer or Die (Michelangelo Antonioni)
Scripted by Tonino Guerra and Anthony Burgess, it was to star Debra Winger alongside Mick Jagger or Richard Gere or Giancarlo Giannini as an architect. Amy Irving was cast at one point as a Catholic novice.