Ok, this is a RUMOR, but it’s one that semi makes sense, so we’ll run with it.
One of the biggest wild cards in the MCU is Hulk, who’s only appeared (recently) in the Avengers movies (and the end scene of Iron Man 3…we don’t count that). And obviously, especially given the events of Age of Ultron, he’s still a big threat. BUT, he willfully went into seclusion at the end of the movie. So…he’ll be left alone, right?
Well, one rumor states that a new Hulk will arise to make the original one come out. In this case, they’ll arrive in Civil War, and the Hulk will be none other than Red Hulk, aka Thunderbolt Ross, who’ll appear for the first time since The Incredible Hulk solo movie (when Edward Norton was Hulk…).
Now obviously, this is not confirmed. HOWEVER, William Hurt IS in Civil War as Thunderbolt Ross. And Marvel has been flirting with the idea of a new solo Hulk movie. Perhaps this could be the catalyst?