And you think a team-based superhero RPG should not be based on the team-based comic books? Okay. We're never going to agree on anything if that's your starting position.
There are severa kinds of Team comics I've seen...
Fantastic Four, X-Men, TMNT: Most of the cast is stable long term, some changes at reboots. Characters are created specifically for the team... tho' certain characters may also have solo series.
The TV "Why the «bleep» are they together?" ones, such as Superfriends/JLA. Spiderman & Friends (Spidey, Firestar, and Iceman).
The team of shared threat: pretty much any of the Secret Wars or "Crisis of" titles. Avengers is clearly this in the movie. Age of Ultron isn't.
Supergirl,
The Arrow and
The Flash crossovers on TV, noting that Supergirl is a different continuity, and the characters discuss the differences onscreen.
Then you have the Unit model: they're part of some agency, and are assigned together, no choice involved. Agents of Shield is the key exemplar on TV. A close variant is the Master and students, such as
New Mutants.
And finally, the family model: in the TV
Arrow show. Red Arrow is Olly's sister. The Canary is his exGF. White Canary is his exGF's sister, and Olly's fling... Plus Olly's Bodyguard sometimes doubles as The Arrow. Two big disfunctional families. Oh, and they all are hunted by the father of the Canary sisters.
Also, Supergirl and Superman teamups. The occasional calling upon the Amazons in Wonder Woman. Thor and other Norse Gods in Marvel. Even, occasionally, teaming up with Loki.
I've probably missed some nuanced subset... I don't right know which flavor Teen Titans is.
But not everyone wants to be the team of professionals of Alpha Flight ot the X-Men, and many wouldn't want to play the ad-hoc team who decide to stay together.