Yes, Red Hulk. Before any of you quit this page hear me out. Red Hulk could be an ultimate addition to civil war or any other film because if there is a battle between both sides of Iron Man and Captain America and the Hulk chose a side than Red Hulk would be on the opposite side because of how much he hates Banner. Sure he would cost a lot of money, but watching him and the Hulk himself fight each other in an ultimate fight of pure strength would be one of the most amazing battles ever seen in the MCU or even any film; it would possibly be even better than Hulk .vs. Hulkbuster.
Who Is the RED HULK?
Red Hulk, also better known as General "Thunderbolt" Ross became the red hulk after the events of World War Hulk when HULK/banner was struck by the Cathexis Ray which he and many other people were exposed to its radiation. While Banner was in custody at Gamma Base, Ross sided with M.O.D.O.K and the Leader so that they may get rid of the hulk once and for all. But, leader and modok had a different plan they use the new Cathexis ray on Ross and it transformed him into the Raging Red Hulk, a being just like the hulk who instead of getting stronger when angrier, gets otter when angrier. At one point, he even absorbed the hulk out of Bruce Banner for a brief moment of days.In the comics the Red Hulk has fought and/or killed (different times or universe) many heroes and villains and monsters alike such as Hulk (duh!), A-Bomb, Spider-Man, Thor, She-Hulk, Iron Man, The Watcher, Wendigo (killed), Valkyrie, Thundra, Storm, Invisble Woman, Hellcat, Tygra, Black Widow, Spider-Woman, Abomination (killed), Psycho-Man, Terrax, Silver Surfer, Dormammu, Namor, Tiger Shark, Doc. Strange, Baron Mordo, Grandmaster, Deadpool, Punisher, Wolverine, Domino, Red She-Hulk, Archangel, Leonard Samson, The Thing, X-Men, Red Ghost and his Apes, Android, Hulk/Terminator, Avengers (Hulked Out), The Intelligencia, the Omegex and many more.
How he could tie in?
General "Thunderbolt" Ross has been confirmed to have a part in Captain America : Civil War, possibly for the hulk or even to become the red hulk himself. What could lead to his appearance is Red She-Hulk, A-Bomb, The Inteligencia and much more. If he happens to even be mentioned it could lead to a world war hulk film which would be so amazing.