A lot of Superman's key attributes are already distributed among the best Avengers, so the team doesn't really need him.
Having said that; Supes will ALWAYS be the MVP if he actually shows up in the presence of any other comic characters. That's just part of who he is. It's the same reason why he will always win crossover battles.
Superman joins the avengers then takes a role of leadership, makes some feel uncomfortable and then realizes he can do alot more being solo and being 'on call' if needed as a part time avenger. Maybe team jump helping others with dire threats and if he finds anything close to a near permanent role on Any team then he takes silver surfers spot in the Defenders.. Surfer is usually busy out in space anyway. Being on any team permanently_ in Marvel U most likely wouldn't work out for him unless he maybe has a hand in making his own which is cheating lol