Did you know Will Smith was originally cast as Neo in The Matrix before Keanu Reeves took the role? There are a ton of films out there where we could have seen different actors in different roles. The studios would officially cast one actor, then that actor would drop out only to be replaced by another actor that we associate that film with today. This happens all the time in Hollywood.
But here is a great collection of movie poster designs of popular films that give us an idea of what some of these movies would look like had the original casting not changed. Some of these might surprise you, enjoy!