Captain America: Civil War may just be the beginning.
Talking about next year's Avengers: Infinity War, Marvel Studios president Kevin Feige was asked if the Guardians of the Galaxy might fight the Avengers before teaming up. Fandango reports he chuckled before saying, "All things are possible…"
Let's just say it right now: Do they really have to fight?
Certainly, there's no end of comic book inspiration for this development. In Marvel's comics, there's almost no way that a meet-up between superhero teams can't end in a fight between them; the first meetings of the Avengers and the X-Men, the X-Men and the Fantastic Four, and the Avengers and the Defenders all ended with conflicts, and even today, Marvel has had two comic book Civil Wars in addition to series called Avengers vs. X-Men and Inhumans vs. X-Men. Those suspecting that the problem might really be that the X-Men are especially disagreeable - well, there might be something to that, really.
By contrast, DC's Justice Society and Justice League skipped straight to working together on their first meeting. 2003's JLA/Avengers series, co-published by DC and Marvel, set each company's premiere super team against the other thanks to cosmic manipulation, while also suggesting that perhaps Marvel's fictional universe is just plain more disagreeable than DC's. Perhaps that explains all the fighting.
When it comes to movies, however, 2016 was a rough year for superhero relationships. Outside of the last Captain America movie, there was also Batman v. Superman: Dawn of Justice and the sight of various mutants beating each other up before forming a new X-Men team in X-Men: Apocalypse. (There was also Deadpool snarking it up with Colossus, but that's kind of its own thing, really; at least there, no one was trying to really hurt each other.) With all that tumult, distrust and violence going on, can't we all just agree that we've had our fill of superhero-on-superhero fights for a long while?
I get the appeal of the trope; it's superhero as sports, and getting to see whether "your" team is better than the other teams. But as a narrative device, it's a dead end, useful for visual effects sequences and little else. The audience all knows that they're going to team up and take on the bad guy eventually, so maybe it's not worth wasting time, even if there are two full-length movies to fill this time around ... and, honestly, it was already beginning to feel a little played out last summer.
It's possible that any conflict between the Avengers and the Guardians will be limited to snide one-liners and side-eye between Rocket Raccoon and Tony Stark. Indeed, that would be the preferable outcome, really, unless Marvel wants to really go for it and have Peter Quill and Captain America have a dance-off to prove their team's superiority. But if, as Feige says, "all things are possible," wouldn't it make a nice change to have "good guys acting like adults and just doing their jobs already" as one of the options?