4. Black Widow
Black Widow is a good friend and colleague of Captain America's, but she'll actually be a member of Iron Man's pro-registration team in Captain America: Civil War. As a result of all this internal conflict, she's all but guaranteed to get a good amount of screen time. She'll act as a mediator between the two teams, whilst also making it clear that she's prepared to fight against her friend if she has to (unless, of course, she turns out to be a mole for Cap, which is entirely possible). The fact that Hawkeye will also be on the opposite side to her in this movie adds another interesting element to her story. She's going to be conflicted and is guaranteed some great fight scenes (she'll probably have one with Sharon Carter as well) and she may even end up switching sides before the end. Essentially, aside from the Sokovia Accords and the Bucky Barnes situation, her arc will be one of the movie's most important. Scarlett Johansson does have third billing, after all.