Looking through the film annals, there are lots of movies that were almost made - and some that were only vaguely considered - that could have been incredible. George Miller's Justice League Immortal for instance. Or Hitchcock's Kaleidoscope. Or Guillermo Del Toro's... well any number of his cancelled projects. But they were destined not to be and there's a sort of tragic romance to that.
Then again, not all cancelled movies end up entirely cancelled. Sometimes, because artists can be inspired by things that were never completely finished, bits of those movies ranging from characters to little concepts to entire scripts can be rescued from the bin and end up in other projects entirely.
It's not ripping off if the thing you're reusing doesn't exist, right?
10. Batman: Unchained
The now infamous planned fourth movie in the Burton/Schumacher series (because it genuinely is all one continuity, somehow), Batman Unchained went by the completely fabricated title Batman Triumphant for a good while.
It was greenlit when Warner Bros saw the rushes for Batman & Robin and inexplicably thought it looked great and it would have featured Batman being mentally tortured by Scarecrow teaming up with Harley Quinn (the original Joker's daughter) to get revenge on Batman. That would have seen Scarecrow use his fear gas to make Batman hallucinate his past villains, including Jack Nicholson's Joker and The Dark Knight having to conquer fear.
How It Lived On
In video game form, in part and also partly in Christopher Nolan's movies, whether directly or not.
The Scarecrow elements of the plot - and indeed the idea of the Joker appearing as a hallucination - was reused for Batman: Arkham Knightand Batman Begins took on some elements of the film's ideas of fear and conquering it.
Even more specifically, script writer Mark Protosevich planned to have Bruce Wayne travel to Bali at the end of the movie where he'd learned that there was a cave full of bats that monks would enter to show they had conquered fear. In Unchained's script, Bruce would enter and bats would swarm all around him as the final shot - an image that Nolan later used in Batman Begins.