5. Vision
Vision has been rumoured to appear in the Marvel cinematic universe for years now. Vin Diesel was said to have been playing the character at one stage - which obviously won't happen now, given his Guardians of the Galaxy casting as the tree-like Groot - and Phil Coulson was even predicted to become Vision after his "demise" in the first Avengers movie (and Tony Stark's reference to him having dated a cellist, which parallels comic book Vision's backstory) - and that's obviously not going to happen now that we've seen him revived (in human form) in Marvel's Agents of S.H.I.E.L.D television series. However, Vision was an essential part of the Age of Ultron comic book arc and he is a must-include element for the upcoming movie. Vision is a creation of Ultron's who ultimately becomes a hero and a member of the Avengers. In the Age of Ultron comics, he is being used by Ultron as a conduit through which Ultron is punishing humankind from the future. It would be a neat way to introduce the character (even though, by this point in the comic books, he is long established as an Avenger) and then have him taken in by S.H.I.E.L.D so that he can be used for the purposes of good in the future - particularly alongside whichever Ant-Man ultimately joins the Avengers.