2. Captain America
How Would It Change The Franchise? The Death of Captain America was a huge money-maker for Marvel, so who's to say they wouldn't try it again? If Cap is killed by Ultron, it could lead to a fracturing of the Avengers. It could also lead to Sebastian Stan's Bucky, who we already know is coming back as the Winter Soldier, taking up the shield. In the comics, Bucky served as Captain America for some time, and Sebastian Stan did initially audition for the role of Cap. Marvel's willingness to adapt the Winter Soldier story-arc shows that they're more than happy with mining more recent stories for their films. Plus, Cap has dropped off before. He's been killed prior to that story-arc, and other people have taken on the costume besides Bucky. For example, John Walker wore it for a while, later becoming the USAgent. And in today's divisive political climate, having a more right-wing Captain America might play well in the Bible Belt.
Will It Happen?Again, this is one that's a real possibility. Chris Evans, like Hemsworth and Johansson, has reportedly had some difficulties with Marvel over future film negotiations. If Marvel decides he's not worth the trouble, the Death of Captain America would make for a shocking turning point in the film and open up the door to future Captain Americas, like Bucky or Walker.