There is one thing that is better than a Black Widow Movie and that's 2 or 3 of them. And there is no reason why not. If you look into the comic version of Black Widow she has done everything from leading the Avengers to picking up Thor's hammer and gaining his powers.
Remember that scene from age of ultron? Where the avengers were taking it in turns trying to pick up Thor's hammer, and she was the only one that skipped her go. Maybe that was a little suggestion that the MCU is set in the same universe as the what if comics! Meaning that Thor will die at some point and the fate of humanity will fall down to her and the other less powerful avengers. She picks up Thor's hammer and uses it which also gives her a costume change.
Do you really want to see a Black Widow movie, or do you want to see a couple? I would like to see at least 3, very unlikely but if done properly she could be as big as iron man, Thor and Captain America. It would be cool to give a character that is always being overlooked more power than she knows what to do with.
I'd like her to be the one to save them all.