Batman vs. Superman
This superhero crossover was close to production back in 2001, with a script written by Fringe and I Am Legend scribe Akiva Goldsman. It was very, very dark. It would’ve featured a grizzled, retired Batman left alone after all his friends are dead. As for Clark Kent, we’ll find him after a messy divorce with Lois Lane. The two are tricked into fighting each other early, but would eventually team up to take on a conspiracy masterminded by Lex Luthor.
Instead, the studio opted to split the two characters for separate franchises in an effort to double their profits (at least it worked for Batman), and didn’t think the movie-going public was sophisticated enough to differentiate a potential standalone Batman vs. Superman from different, separate films with each character.