Hmm, I spent some time looking at canceled films and holy shit, DC has canceled more than they've ever made. x) Too bad they didn't cancel more of the recent ones. Specifically, anything made by Zack Snyder. Anyway, after writing that first sentence, I've spent a lot longer
looking at canceled films. I'm not surprised there is a ton of them, but goddamn. Don Quixote must be cursed because holy shit, many attempts have been made and stopped. They really shouldn't bother, Wishbone did it best.

Otherwise, lots of shitty animated sequels, lots of unnecessary remakes and reboots, lots of inane adaptations and so on.
The standouts would have to be Spore and The Sims for video game movie adaptations. Like, what the hell? Those games do not lend themselves to be movies very well. The remakes of Akira and the Yellow Submarine should stay in the grave. Akira was hard enough to adapt as is, I don't have any faith in a live action adaptation meant for Hollywood. Chronicle was inspired by Akira and was good enough as an Akira film, just leave it at that. The Yellow Submarine was a 3D film and sounds like a total disaster. Robert Zemeckis was the director and while he has made some fantastic films, I feel like he hasn't made a good one in 25+ years, especially not his 3D movies.
Finally, the entirety of the Universal Dark Universe. They banked on it being a thing so fucking hard and it's immediate death after The Mummy will always bring me joy. It's just a bad idea to begin with, but the involvement of Alex Kurtzman and Chris Morgan as the leads behind the shared universe is the most terrifying part. I will also never bemoan the icky cultist creep Tom Cruise getting denied another cash cow franchise. Like he really needs more money to inject into Scientology. Ugh.