I really don't see anyone taking the mantle of Captain America, Iron Man and Thor especially when the likes of Captain Marvel, Dr. Strange, Spider-Man, Ant-Man and Black Panther are headlining their own solo films right now and already appearing in the Avengers movies. And there's still the Eternals, the X-Men, the Fantastic Four, Shang Chi on the way. There's no need for legacy characters in these movies right now.
The comics just constantly use/create legacy characters because they are easier to sell than to create a character that isn't highly inspired by an existing character which imo is kinda lazy, they have also been using the characters for decades and they are simply running out of ideas. While the Marvel Stud10s films are still new.
As for Hawkeye, I seriously doubt Kate Bishop could replace Clint in the films, especially she's being introduced in a tv show which automatically limits her audience.