We're heading toward the end of the current phase of the Marvel Cinematic Universe with Avengers: Infinity War and the follow-up Avengers 4. Chris Evans has explained why he felt compelled to finish out the story even though he wasn't supposed to. Originally, his contract with Marvel was only for six films, which meant it would end with Avengers: Infinity War. Recently, he spoke about why he decided to extend that contract for one more movie. He thought it just made sense to hang around until the story was over.
I had six films in my Marvel contract, so I could have said after the third Avengers I was done, but they wanted to make the third and fourth Avengers films as a two-parter. They said they had so many other characters to fit in -- Guardians of the Galaxy, Black Panther, Captain Marvel, Doctor Strange, Ant Man -- and couldn't get them all into one movie. [I agreed] because it made sense. It's going to wrap everything up.
Marvel has always claimed that the company doesn't actually have its cinematic universe planned all that far in advance, and Chris Evans' comments to The Telegraph would seem to coordinate with that. Evans' original contract was for six movies, which shook out to three Captain America titles and three Avengers movies, however, since the decision was made to make Avengers 3 and Avengers 4 essentially two parts of one story, Marvel then needed to get Evans on board for the extra movie. Originally the two movies were being called Infinity War: Part One and Part Two and while that's no longer the case, it's clear that the two films are still connected.
It's certainly true that these two Avengers movies are going to be full of characters. One of the most difficult things the movies will have to accomplish is figuring out how to balance those characters so that they each have a reason to be in the movie, and not simply be a cameo for a cameo's sake. We can assume that Chris Evans will have a significant role in one or both movies.
Of course, talking about Chris Evans extending his Marvel contract once makes me wonder if he would be interested in doing it again. The two biggest actors in the MCU currently, Evans and Robert Downey Jr, are officially done with Marvel once Avengers 4 wraps_. _ With so many other characters to play with, odds are that the focus of the MCU will just shift elsewhere, though if one or both actors were willing to hang around, one assumes Marvel might at least be interested, depending on how the next movies pan out.
Of course, Marvel's Kevin Feige has already made comments that the MCU could be going through significant changes following the end of this Phase, so who knows what's going to happen going forward. Chris Evans will play Captain America again in Avengers: Infinity War in May of 2018.