After the release of Guardians of the Galaxy, it was obvious that the idea of "backstorying" was pointless. No one really cared, or needed, an in depth back story that would take up a third of the movie, especially with several main characters, this would have been impossible to do and tell the story James Gunn wanted to.
And now, with the release of Ant-Man and the soon-to-be released Captain America: Civil War and Thor: Ragnarok it opens up a new venue for the Marvel Cinematic Universe. No longer is it required to have back stories.
For instance, it was not necessary to have a Black Widow, Hulk or even Hawkeye stand alone movie. Nor was it necessary to have in-depth back stories for Falcon, Vision, Scarlet Witch or Quicksilver. So the question now is this:
In the next steps and next movies, we could see introductions of new characters and integration of older characters immediately. What could be looming close are movies that just incorporate characters to drive the story. For instance, Spider-Man will be in Civil War, and there are a flurry of theories that Hulk will be in Ragnarok. In the near future we could, and probably will, be seeing more of these characters, being introduced as secondary heroes with primary roles (like they did for Phase 2 with Scarlet Witch, Vision and QuickSilver; and what they did for Phase 1 with Hawkeye and Black Widow).
What Marvel has shown is that, indeed, backstory is not necessary. You may say that characters that people are familiar with don't need back stories, but I had to Google "Falcon" to see if he was really in the Marvel Universe to be familiar with him. What Marvel has done is something impossible. They have created a system in which the audience wants to watch their movies because we know they are fun. Not only that, but the continuity that they created with the first Iron Man movie with Nick Fury breaking into Tony Stark's house set the stage for a huge universe, a universe that now is free to do almost anything.