So here's a question:
Will "Avengers" continue to be used as the title of the team-up movies even after FF4 and others are introduced into the universe? I assume that Marvel will want to continue to make big team-ups, but there may not really be an "Avengers" team that is as iconic as the current 4-movie roster. With Cap, Iron Man, Thor, Hulk, and others potentially gone after this movie (or at least the end of the next phase), do you think the Avengers brand will turn into their big event movie name?
My personal hope is that they retire it for a few years.
There are a ton of Avengers teams in the comics that they could touch on, and I hope there still exists an Avengers team in the MCU, but I think they could have so much more potential going forward if they don't center the MCU around the Avengers so much.
For one, I think the Fantastic Four really have more potential as a driving force of Marvel stories like they did for decades in the comics. Granted, they aren't always going to be the center of every story, but by their nature of being explorers at the forefront of all the discoveries of the Marvel universe, they are in a position to have the potential "Phase 4" era of the MCU be driven by the Fantastic Four opening all the doors.
Another reason is that there are events in the comics that don't have to necessarily revolve around the Avengers like Civil War was one, Annihilation is another, Secret Wars wouldn't necessarily be revolving around the Avengers, etc., so having their main "event" movies center around one ensemble team franchise that they keep adding characters to they need to juggle just becomes too hard to manage as well as limits the stories they can tell.
It will be hard to swap out the original roster for a new team, regardless of how well-received the new characters in the MCU has been. That's not to say that I don't think they can do that can keep the momentum more or less going, I just think going a more unexpected and unconventional direction would be more exciting that would allow the MCU to expand and increase interest rather than fighting the uphill battle of trying to top and recreate their greatest hits for the first 3 phases. They could have the original Defenders line up (probably with a different name) or maybe the Revengers make a come back. The Dark Avengers/Thunderbolts could be another direction a team could go that maybe gets it's own movie. The Ultimates is a team I'd personally like to see in the MCU just from being a bad ass lineup of powerhouse (and diverse) characters with a more cosmic focused Avengers-like team. Maybe a full-on Secret Avengers or Young Avengers that don't have to be treated like an event movie, there are so many places they can go.