7. Secret Invasion
There's a lot of confusion surrounding whether or not Marvel is able to use the Skrulls in their movies, but it wouldn't exactly be hard to come up with a new race of shape shifters if they can't. Secret Invasion was a great idea poorly conceived by comic book writer Brian Michael Bendis, but introducing aspects of it into the Marvel Cinematic Universe is a must. Avengers: Age of Ultron could set this up by making fans questions the motivations of one of the members of The Avengers; for example, what if the Tony Stark who fell from that wormhole in the first movie isn't the same one that went through? The reveal in Avengers: Infinity War that Thanos has a spy placed on Earth could be a great moment whoever it is!