18. Magneto, Wolverine, And Namor (Captain America: The First Avenger)
Because Captain America is a bit cheesy, Marvel Studios were terrified about adapting him for the big screen. The studio were so worried that Cap couldn't work in a serious film, they strongly considered making the movie into a comedy.
When that idea was scrapped, Marvel had another idea to draw in audiences; cameos. Because, when in doubt, rely on fan-service. Since the story takes place during WWII, Marvel toyed with the idea of having Wolverine appear as a soldier. Another idea was for Cap to save some concentration camp prisoners including a young Magneto.
In the film, Cap squares off against the Red Skull but he was also meant to battle two other iconic villains. As is depicted in the comics, Bucky would have been seemingly killed by Baron Heinrich Zemo. (His son, Helmut Zemo, was the antagonist of Captain America: Civil War.) The studio also wanted to showcase the leader of Hydra; Baron Wolfgang Von Strucker. He eventually appeared in Captain America: Winter Soldier and Avengers: Age of Ultron.
Namor the Sub-Mariner, who is the king of Atlantis and one of Cap's oldest allies, was also meant to cameo but the studio couldn't secure the rights for the character.