I really don't see how they can do Civil War without there being a broader base of heroes in the Marvel Cinematic Universe.
The core point of contention is the registration act and the question of government sanctioned superhumans vs. secret vigilantes. Right now the only superhuman heroes in the Marvel Cinematic Universe are the Avengers and until Age of Ultron hits only Thor, Hulk, and Cap have legitimate super powers! (Widow and Hawkeye are very very talented humans and Iron Man is a suit)
That is exactly why Iron man wanted them regulated. So that might work pretty well.
If you have read the news about marvel lately, they are going to be introducing quite a few superheroes. Dr. Strange, Black Panther, Daredevil (series but Daredevil played quite the part in civil war so i expect a crossover) Luke Cage, Mockingbird and whoever the hell they feel like.
It doesn't really matter that it's not going to be a carbon copy of the comic series. The one character they do not have plays a very critical role in the civil war story though....Spiderman. He is a big PoV character in the entire civil war story because he gets goaded by Iron man to reveal his identity, pretty much the only superhuman who actively tried to hide it. That is the big problem i see with civil war being filmed.
How to trigger the civil war? simple. Hulk goes apeshit. Iron man blames banner, Cap protects Banner. There ya go, you just fixed the problem of introducing the new warriors, various cannonfodder and you can erase the entire wolverine storyline. Mutants weren't a very big part of the civil war because Emma Frost basically told both cap and Iron man to fuck off. Punisher can be used as the guy who makes Caps crusade look like a failure. And Crossbones can still kill cap in the end. Fantastic four can easily be taken care of not using them because it's a sidestory between a house divided and the impact of the superhuman registration act. Let go of the Negative zone but use some far away special prison based on earth and you can still keep the prison break in by cap to free all the ones who got imprisoned.
I just hope they do a Thor movie after civil war, where Thor comes back(don't need him during the story except a clone for like 1 scene) and Iron man threatens him, then Thor pimpslaps Iron man (coolest comic book moment ever) and Thor tells him to go fuck himself.
So all in all you need a small core as a PoV,(Cap, Iron man, spiderman,1 Scene of Thor, some newly introduced heroes and some no name cannonfodder in a slightly familiar outfit.