In the aftermath of the Civil War, Steve Rodgers, after being taken into custody, was assassinated by his girlfriend and loyal S.H.I.E.L.D. agent Sharon Carter, who was mentally possessed by agents of the Red Skull. Steve's friend James "Bucky" Barnes targets Tony Stark, blaming him for Steve's death, until Tony finds a letter from Steve saying that the legacy of Captain America must continue. Tony then offers to instate Bucky as the new Cap, and he accepts.
Could this be where Marvel is headed?
Most of us know of Joss Whedon's infamous habit of killing off beloved characters that we have come to know and love (I'm looking at you Wash and Book). He could very well bring back that trend in the third Avengers movie. Since Robert Downey Jr. has only been booked up until the third, I predict the final battle with Thanos will take place then, and you know Whedon won't let it slip unless a major character dies. Many think that Loki is going to be the one killed, and very well may be, but my bet is Cap.
If you don't believe a theory, have a look at the evidence. The first Cap movie featured none other than Bucky Barnes and some Agent whose last name escapes me. Oh yeah, it was Carter. Not Sharon Carter, but Peggy. Although Emily VanCamp has been confirmed for Captain America: The Winter Soldier as none other than Sharon Carter.
For those of you that do not yet know, Bucky Barnes is the Winter Soldier. That means that the second Cap movie (that comes out on April 4th) will be focused on Barnes himself. How much do you want to bet that Bucky is going to snap out of his evil kick and start up as a hero, maybe the next Cap?
Another original Cap character will be reprising his role in the sequel, Toby Jones as Red Skull's doctor buddy Arnim Zola. Although Red Skull isn't going to be in it personally, the appearance of Zola teases Skull's return for the next Cap, and possibly the third Avengers.