What makes a superteam great?  More specifically, what makes the Fantastic Four so stable?  And what makes the X-Men so fractious?  When you're putting together a team of new super heroes, take some lessons from the past and build the kind of team you want, stable or otherwise.  On a stable team:

Everyone is good at something special.   The Fantastic Four are the best in the world because each of the succeeds at something different.  This shows up their ability skills:  Reed is the smartest, Ben is the strongest, Johnny's the fastest, and Sue's the steadiest.  There's never any question on this team as to who's going to handle any given problem.

Motivations are in synch.  With the Fantastic Four, everyone is behind the goal of exploring and protecting the universe.  But the X-Men have suffered because they are centered around an ideal that not all of them share.  Professor Xavier is dedicated to the belief that humans and mutants can co-exist peacefully, but not all the X-Men are willing to take constant xenophobia without lashing back.

Leasdership is strong.  The Fantastic Four are led by Reed Richards, pure and simple.  But the founder of the X-Men isn't always around.  Xavier routinely leaves the mutants in the hands of less idealistic teachers like Cable and The White Queen.  Even Cyclops, Xavier's nominal second in command, can't take care of a half-dozen superteams.  So the message gets muted, and tensions brew.

Villains are omnipresent.  The thing that keeps a superteam functioning best is constant prompting to action.  The Fantastic Four's public presence guarantees that assaults on their headquarters will break up the routine.  Nothing builds solidarity from within like attacks from without.