I think the problem is that phase 4 and 5 have had way too many shows and movies to be represented in two Avengers movies.
And why would a street level character like Echo be part of this?
Aside from Agatha/Echo, there's also Wonder Man, Werewolf by Night, Man Thing, Moon Knight and She Hulk, all have yet to appear in the movies.
From the multiverse saga movies, these are the characters returning in Doomsday
Black Widow - Yelena/Red guardian
Shang-Chi - Shang-Chi
Doctor Strange 2 - Professor X
Thor 4 - Thor
Black Panther 2 - Shuri, Mbaku
Ant-Man 3 - Ant-Man
Marvels - Beast
Deadpool & Wolverine - Gambit
Brave New World - Sam, Joaquin
Thunderbolts - Sentry, US Agent, Yelena, Red Guardian, Ghost, Bucky
F4 - F4
Spider-Man being saved for his own movie/Secret Wars. Star-Lord for Secret Wars (I guess). Then that just leaves the Eternals.