The only way an all female marvel movie would work in my opinion would be if the opening scene are all the avengers fighting some villain and then something changes reality and all the men disappeared. Its a common theme and easy route to get all the females together but any other route wouldnt make sense to do it alone. Black widow would be smart enough to call cap as backup. Scarlett has vision to complement her. Maria Hill is working for iron man if i remember right. Just my thoughts though.