13. Batman Unchained
The Movie: During production of 1997's Batman and Robin, Warner Bros. executives were impressed enough with the raw dailies to greenlight a third Batman movie under Joel Schumacher's helm, entitled Batman Unchained.
The film would've had George Clooney, Chris O'Donnell, and Alicia Silverstone reprising their roles, while Scarecrow would appear as the primary villain, using a fear toxin to resurrect The Joker as a hallucination, with Warner Bros. hoping to lure Jack Nicholson back for the part.
Also, Harley Quinn was to have a small supporting role as The Joker's daughter, weirdly.
Why It Got Cancelled: Batman and Robin was a box office disappointment and so enthusiastically torn apart by critics that a sequel was deemed untenable, especially as an embarrassed George Clooney declined to return.
It all worked out in the end, of course, for while Batman and Robin killed the Caped Crusader's cinematic tenure for almost a decade, it led to the radical Christopher Nolan reboot Batman Begins, and the two unforgettable sequels that followed.