What was the biggest missed opportunity in film history (such as a director dropping out of a project or an actor being replaced)?

There's a famous clip of David Lynch recalling meeting George Lucas because he was approached to direct Return of the Jedi (which he had no interest in), and the potentially mindblowing clusterfuck that could've been Lynch's Star Wars got me thinking: are there other such examples of huge missed opportunities in filmmaking history that would have been amazing to see? via /r/movies https://ift.tt/3o0UcqA