I'll only mention stark and CA, some of it holds true for the other guys but they are all I can think of at the moment for even possibilities. Superman's got his hands full.
They still have to work wit the system, they aren't kings. CA seems like the best man, but can he play the politicla game with his sense of morality? Can he bend enough, but not break. Stark is kind of the flip side. He's smart, goal oriented, but he's an ass. He does things his way and screw the rest. He has a political system to work in. He can't laugh at everyone and pull out a gadget and blow off congress, supreme court, etc. His inventions plus goverment backing could probably go a long way to some real remedies for society ailments. If he doesn't piss everyone off long enough to get impeached or assainated. His cavalier attitude in international politics would laso be playing fast and loose with the whole country, not just risking himself in his armor.