9. Wolfgang Petersen's Batman V Superman
With the Dark Knight on an enforced cinematic hiatus after the disastrous Batman & Robin and Superman still in the wilderness after The Quest for Peace disaster over a decade prior, Warner Bros. decided to kill two birds with one stone in 2002 and reboot the iconic superheroes in the same movie.
A director was hired, the script was written, a summer 2004 release date was announced and offers went out to Christian Bale and Josh Hartnett to play Batman and Superman respectively. Then the studio suddenly got cold feet, deciding to focus on separate movies for the characters, and the whole thing quietly dissolved.
Recent history has shown that a Batman V Superman project is guaranteed to be a huge hit, whether its any good or not. As for how it could have changed comic book movies forever? Take your pick.
With Superman Returns off the table Bryan Singer may have stuck with the X-Men franchise and avoided whole Last Stand debacle, Christopher Nolan would never have gotten his hands on the Dark Knight and reinvented the entire comic book genre and most interesting of all, the two DC icons would more than likely be spun off into vaguely-connected solo movies before the occasional team-up, years before Marvel Studios even existed.