This article from Mental Floss compiles some of the Hitchcock projects that didn’t quite make it all the way to the screen. I’ve read a little about the pre-production for The Blind Man before, which is the most compelling of the list for me.
The Blind Man
In 1960 Hitchcock and legendary screenwriter Ernest Lehman began to work on a script called The Blind Man about a blind pianist who regains his sight after receiving an eye transplant from a murder victim. Hitchcock envisioned Jimmy Stewart in the lead role, and one of the film’s main scenes was to be set in Disneyland. That’s where the trouble started; Walt Disney had seen Psycho and truly hated it. Disney reportedly wouldn’t let Hitchcock shoot in his park, so the project died.