I like that idea. Primarily because it means many Avengers movies. But its a minimalist ideal when you think about it. They should really be striving to keep our main cast as long as possible (short of keeping them around with grey hair). The core four should hold out for at least a trilogy of Avengers and MCU phases.
Plus an Avengers without Iron Man/Hulk/Thor, while possible, seems to me... somewhat dull. Personal opinion of course.
I think (or hope anyway) that they could build up enough excitement around other Avengers that would equal the excitement around Iron Man/Hulk/Thor and Captain America. Once these actors leave the parts others could take over. Thats another reason for the staggered rotating roster. So that when Thor leaves the show, and when Iron Man leaves, and when Cap leaves, people wont feel like they are being forced into liking the new guys taking over. If characters are introduced while the cast is still there its less forced. If Ant Man is brought in in Avengers 2, then by the time everyone is gone in Avengers 4 and only he and Wasp remain the audience has already gotten to know him enough to see him as part of the team and not necessarily taking over for Captain America or the rest. Sort of like on Law and Order, Detectives would come and go, new ones would become the partners of the older ones who had been there longer. But then the older ones would leave and the younger ones would become the new older ones and get newer partners. The Lawyers would get new assistant/partners all the time. The cast would rotate and change and the franchise was kept alive for many many years. They could do something similar.
I think given the right actors/plots/dialogue, characters like She-Hulk, Captain Marvel/Carol, Luke Cage, Falcon, Black Panther, Vision, Wonder Man, Bucky/Cap, could all become big fan favorites.
(Also many of them would get their own solo films, and others would be introduced in previously established fan favorite characters like Bucky and Falcon in Captain America 2. Plus some could be introduced in 15 minute shorts before each Marvel film.)
But I do agree that there is a risk of audiences not connecting to the newer characters the same way.
My idea is a very basic blue print. I reasoned that an actor like Chris Evans probably wont want to become known for just Captain America, and once he has fulfilled his contract of doing 3 Avengers films, 3 solo Captain America films, and 3 cameos in other Marvel films, he would leave the role. Downey Jr. is already ready to leave Tony Stark behind, not sure if he will do 2 more Avengers films but probably will. And I am guessing most of the actors would do the same. That means by Avengers 4 would would have an entirely new cast of characters, unless some of them stuck around. Or maybe Marvel could save the contracted cameos for Avengers 4 so that Stark or Thor or Cap could pop up in Avenges 4 for a little.
but the basic idea of my... idea, is that to keep the crowds interested we slowly introduce new characters into the background of the Avengers and rotate them to the front when the originals leave the movie. Only when needed, if Thor plans to stay there is no reason to kick him out. But still bring in other Avengers like Namor or Valkyrie, or Giant Man and Spider-Woman. I am not saying my idea is perfect, really its just me trying to build the franchise I want the Avengers to be so that I could see other Avengers like Ant Man, Wasp, Yellow Jacket, Beast, Namor, Black Panther, Hercules, Wonder Man, Vision, Scarlet Witch, Luke Cage, Mockingbird, War Machine, etc, as Avengers, and not wanting the Avengers to end or relaunch after Avengers 3.
