At the end of Avengers: Age of Ultron, Bruce Banner/Hulk, played by Mark Ruffalo left the Avengers to fly out in a stealth jet by himself. No one really knows what happened to Hulk/Bruce by the end of Avengers: Age of Ultron. Although it appears that Hulk's actions are hinting at Hulk holding some level of intelligence which gives credence to the theory that Bruce Banner and Hulk are beginning to meld their minds to allow Hulk to retain some or all of Banner's intelligence. Hulk was able to close down the HUD screen transmitting to Black Widow, as well as coordinate the stealth-jet in a specific direction after the climax of Age of Ultron which proves that Banner/Hulk are becoming one. If this theory is proven to be true, than we may see Hulk fighting alongside one side in Captain America: Civil War, most likely alongside Captain America in light of his joint philosophies towards privacy and secrecy. Mark Ruffalo isn't scheduled to appear in Civil War but a recent casting slip by a co-star, Anthony Mackie of Captain America: Civil War cited Mark Ruffalo as being cast in the movie. It might have just been the case of a slip-of-tongue but this slip could mean that Mark Ruffalo will be playing Hulk in the upcoming Captain America: Civil War. Regardless of Mark Ruffalo being cast in Civil War, it's William Hurt's inclusion as General Thunderbolt Thaddeus Ross that will have greater impact on the MCU if he fulfills his comic-book destiny. In Marvel Comics, General Thunderbolt Ross is famous for being one of Bruce Banner/Hulk's greatest adversaries. General Ross seeks out a way to duplicate Bruce Banner's Hulk transformation, with some extensive research, General Ross is able to duplicate the Hulk/Banner formula and transforms himself into the Marvel villain, Red Hulk. General Thunderbolt Ross takes on the role of Red Hulk in secret, he doesn't reveal himself to be Red Hulk until he confronts Bruce Banner. Within the MCU, we've only seen General Ross in The Incredible Hulk, and now he's been seen in recent promos for Captain America: Civil War. Going along with the information given away by General Ross' role in Captain America: Civil War, taken from the trailers for the film, General Ross is proposing the Sokovia Accords which mirrors the Marvel movement known as The SuperHero Registration Act, which required all enhanced individuals to register and be held accountable for their actions. If General Ross is indeed proposing The Sokovia Accords, the Sokovia Accords will most likely be brought about based on the Avengers' actions in Avengers: Age of Ultron. That furthers General Ross' involvement in the conflict of Civil War, General Ross has been known to go to extreme lengths to capture Hulk/Bruce Banner in Marvel Comics, if General Ross is indeed playing some part in The Sokovia Accords with the intent to get Bruce Banner to register, General Ross could have an ulterior motive, possibly to duplicate Bruce Banner's Hulk transformation. These are all speculative theories based on General Ross' inclusion in Captain America: Civil War, the MCU could be following General Ross' comic history to replicate his Red Hulk storyline or the MCU could be creating their own interpretation of General Thaddeus Ross. Although William Hurt's portrayal of General Ross hasn't been given much screen time, there is the aspect of General Ross already having a connecting role to the MCU. It was established in The Incredible Hulk, that General Ross meets with Tony Stark in a bar which could have several implications for the whole of the MCU, one meaning that Stark has been sharing information with General Ross since the events of The Incredible Hulk, and the other being that Stark and Ross have been buddies since the events of The Incredible Hulk, and the two will share some allegiance coming up in Captain America: Civil War. In either event, if General Ross is helping Tony Stark in Civil War, then Tony Stark may have an incentive to assist General Ross in duplicating the Hulk formula which would eventually lead to General Ross turning into Red Hulk. General Ross turning into Red Hulk could only fit into so many contexts if he is included in Captain America: Civil War. For one, Tony Stark would have to be facing Hulk on Team Captain America to require the assistance of an equal force to combat Hulk which is where General Ross could come in to be the antithesis for Hulk. And the other context, could be in the circumstance that General Ross has captured Bruce Banner/Hulk after the events of Avengers: Age of Ultron, then submitting Banner to experimentation to duplicate his Hulk transformation, is unable to duplicate the process without Tony Stark's help; General Ross could use Banner's life as a bargaining chip in a deal for Tony to duplicate Banner's transformation. These are two absolutely plausible conclusions for General Ross' return to the MCU, General Ross hasn't made an appearance in the MCU since The Incredible Hulk which means his return was brought on by a significant event in the MCU. The event in question is most likely Hulk's actions in Avengers: Age of Ultron, which would be enough to bring General Ross' attention on Bruce Banner once again. Aside from the speculations and theories, General Thunderbolt Ross will definitely have some part in the upcoming Captain America: Civil War film, let's just hope we get to see another Hulk on the big screen that'll give Bruce Banner a run for his money when it comes to hulking out.