The Disney+ series look set to introduce members of the Young Avengers, but i think once that team's formed the senior characters from those series might make a guest appearance backing them up basically meaning we'll have an MCU equivalent of the West Coast Avengers made of familiar Phase 1&2 characters like...
Cap/Falcon, Winter Soldier/Cap, Scarlet Witch, Vision?, Hawkeye & War Machine
...They could pop up to aid a new team on the big screen without their Disney+ storied being essential viewing as they have their backstory from the movies, but the audience don't need telling where they've been in between because the series will explain that if it matters.
As for the next Avengers roster on the big screen, As many have said the obvious six seems to be...
Ant-Man, Wasp, Black Panther, Captain Marvel, Doctor Strange & Spider-Man
...but I think we might get a couple of surprises there. Captain Marvel could be occupied with the cosmic side of the MCU too much to be on the team rather than an ally, T'Challa could possibly join her on an Ultimates team and I don't think making their appearances as part of a team does Strange and Spidey justice.
Ant-Man & Wasp definitely.
Black Panther very likey
Captain Marvel, Doctor Strange and Spider-Man .... maybe but by the end of Avengers 5 there will definitely be someone new on the team.
An Eternal or two no doubt but then there's also Shang Chi and other new characeters on their way to consider.