4. Captain America Vs Iron Man
Marvel's "Civil War" was a 20062007 storyline which followed the implementation and consequences of the Superhuman Registration Act - a legislative bill which required the mandatory registration of any person based in the United States with superpowers. Captain America and Iron Man found themselves with opposing opinions, with Iron Man, who had previously tried to halt the act, becoming the pro-registration figurehead and Captain America leading the anti-registration group. Each hero garnered support from a legion of fellow heroes and a brutal war ensued, with Cap and Iron Man battling it out with each other during said war. There were far too many heroes involved in the story to include in a movie, but a scaled down version of the war with Cap and Iron Man leading the sides, or even just the two of them fighting each other over the matter, could work on the big screen. Of course, there has already been a hint of disharmony between the two characters in the Marvel Cinematic Universe, with Cap urging Iron Man to "put on the suit" and "go a few rounds". Seeing that actually happen would be cool as hell.