Multiple Indiana Jones sequels that never happened
No list of abandoned Steven Spielberg projects is complete without a partial reckoning with the numerous Indiana Jones spin-offs and sequels that have been developed, mentioned in interviews, or rumored in the press -- and then disappeared completely. Few franchises have as many skeletons in the closet: There's Indiana Jones and the Haunted Mansion, Indiana Jones and the Monkey King, Indiana Jones and the Saucermen from Mars, Indiana Jones and the Lost Continent, and, of course, a rumored spin-off based around Shia LaBeouf's character Mutt. (When dismissing the prospect of a LaBeouf stand-alone in a 2008 interview, George Lucas joked that it would be titled "Mutt Williams and the Search for Elvis or something.")
Honestly, they're all fun to read about, but none of the hypothetical sequels quite have the same hold on the fandom's imagination as a script written by The Shawshank Redemption director Frank Darabont, who had cut his teeth working on The Adventures of Young Indiana Jones series. In 2002, Darabont hired to write a draft, and turned in a script titled Indiana Jones and the City of Gods, which according to the writer, Spielberg said was "the best draft of anything since Raiders of the Lost Ark." According to interviews with Spielberg himself, George Lucas disagreed. The rest is very complicated (but well documented!) history.
The public goodwill that exists towards Darabont's script, a version of which appears to be available via WikiLeaks, likely stems from how much many people despised 2008's Indiana Jones and the Crystal Skull. As with all the projects on this list, it's often easier to pine for the movie that never happened than to confront the reality of the movie that actually got made. Maybe the City of Gods is where all abandoned Spielberg projects go when they die?