The Russo brothers were interviewed and said the following about the Defenders crossing over with The Avengers
"When we start to serialize the telling of stories is difficult. You have to have a lot of control and focus on the course of history. The films are controlled by a group led by Kevin Feige, so they function as a unit. Other products, even if they are from Marvel, are controlled by others. Then there is the possibility of a crossover, but is more complicated. It is a smaller version of the problem that exists when thinking that Fox has the rights to certain characters, Sony and others. As storytellers, we only have control over what happens in Marvel movies, but everything is possible, Spider-Man (which has the right held by Sony) was possible!"
What do you think?
The Defenders should join the Avengers and this seems promising to me!