Scarlet witch would be a 100 times more interesting than black widow, who really wants to see another film of a hero who can just punch and agility people to death
She'll play Spiderman's new fling in this movie to set up a love triangle with Starlord in Guardians Of The Galaxy 12 which will culminate in a Mexican stand-off in The Avengers 356.
Black Widow can't be that much of a commercial magnet 
