Keep it simple.

His brother corrupts him. Tony Starks gets him arrested. Baron Zemo gives him powers for revenge against him. His conscience takes over and he fights back against Zemo and possibly his brother with help from Tony. At the end he can fall into that "coma" or not.

Next movie, he moves to LA becomes an actor/stuntman because he's too paranoid to become a hero again. While on the west coast, that Gravity dude from the comics attacks LA, Avengers aren't available, so they form the WCA or at least introduce some of the members to stop the threat.

Him and the Human Torch are needed more for the Vision's origin, imo.