Chris Evans doesn't want to act anymore, so I imagine they're going to kill off CA sometime "soon". I'm guessing Stark will rationally deduce the raw necessity for an authoritarian "solution" to managing those with superpowers, CA will oppose him in the name of freedom, and Stark will end up (accidentally or regretfully, maybe indirectly?) killing him. This will devastate and fracture the entire "cast". Maybe Stark and/or other members are killed in the same or ensuing conflict. Maybe the fight propels one or two disillusioned heroes into retirement or hiding. Some of the new faces will be successors aka recastings. You know there's going to be a new Captain America taking up the fight for freedom. Tony's getting old (as is RDJ), so I imagine he'll be looking for a smart young pragmatist to carry on his work.
This is all very inevitable. A lot of the principle leads in these movies are tired of making them, and the studios have pretty much reached the point where changing actors' minds with obscene paydays is verging on threatening the financial viability of the current model. The continued success of the model is going to hinge almost entirely on nailing their casting. RDJ carried the Iron Man franchise. Hiddleston carried the Thor franchise. Together, they carried Avengers. If they can't find similarly magnetic people to fill at least some of these vacant shoes, they'd better find themselves some damn good writers.