Yep Maya (Echo) was a Ronin as well and I believe part of Luke Cage and Doctor Strange's New Avengers. Remember though it's Otto-Peter that is a member of the team, so we have that different dynamic to deal with. I suspect the supporting character thing is exactly why we have this specific team.
Also Monica while being apart of Carol's supporting cast, doesn't mean she hasn't been active as a superhero in it with Carol, which she has been. She just hasn't had a codename I don't think. Carol usually addresses her with first name. I believe part of her story arc in "Captain Marvel" has been getting back to being a hero again so her thread may well lead into "Mighty Avengers".