The big question everyone asked during Phase 2 of the Marvel Cinematic Universe was "Where are the other Avengers?" In Iron Man 3, people wondered how come Captain America couldn't come help Iron Man and Iron Patriot save President Ellis? In Captain America: The Winter Soldier, why didn't Iron Man come and help fight Alexander Pierce, and take down the Helicarriers? To me I don't think that problem will be answered, but I do think it won't be much of a problem in Phase 3.
[Captain America: Civil War](tag:994409) is set to come out May 6, 2016, and will see the Avengers splitting sides and fighting for what they think is right. But what the dark truth is, is that by the end of this movie, the Avengers will be split up, on their own, and probably hating one another.
So in the movies that take place on Earth, and make sense, which are Spider-Man, Black Panther, and Ant-Man and The Wasp, and possibly Doctor Strange, Guardians of the Galaxy Vol. 2, [Thor: Ragnarok](tag:956858), and Captain Marvel. These movies will likely not see other Avengers (it's not impossible, though), and there is an easy answer: the events of Captain America: Civil War have caused our heroes to deeply distrust each other, so when they are in need, they will not want to help them, even if it may end up with many people dead.
This distrust will go on for years. but when the mad Titan Thanos arrives on Earth and threatens to destroy the whole planet in the Avengers: Infinity War movies, our heroes must trust each other once again to stop their most perilous threat. They'll be "Getting the band back together!". Sorry, I had to.
Distrust. That is the key word for Phase 3 of the Marvel Cinematic Universe. And no one can trust another Avenger, so could this lead to how the Defenders will be included in the movies? Say, in the Spider-Man movie, Spidey needs help but knows others won't help him, so he decides to go to Hell's Kitchen to try to get help. There he meets Daredevil, and he helps Spidey with whatever he needs.
And since I'm sure Daredevil, Jessica Jones, Luke Cage, and soon Iron Fist, have all probably heard of each other, what if in Avengers: Infinity War, Spider-Man and the Avengers must protect the world from Thanos, so Spider-Man gets Daredevil to help them, who could potentially bring along Jessica Jones, Luke Cage, Iron Fist, Hellcat, Elektra, Misty Knight, The Punisher, and probably many, many more.