There’s an enormous assumption here that there needs to be a next Avengers film.
The formula Marvel’s developed - individual movies leading to a crossover - is a good one. It works pretty spectacularly. But no one says it should be limited to just going forward with Avengers. They’ve got the X-Men now - which might benefit from a rest - and more importantly they’ve got The Fantastic Four. I’d build the next team around the FF with guest stars and move forward there.
It’s important to know what you have, yes, but it’s also important to take chances and try new things. It’s not like Marvel Studios hasn’t done that already with Guardians. Who they hell were the Guardians of the Galaxy before they were announced as an upcoming movie? A b or c list team of non-powered weirdos.