Speaking with Vanity Fair recently, Neil Marshall stated he is interested in directing a movie with a strong female lead who doesn't have a whole lot of superpowers:
I think I’d like to do a big movie with a strong female lead, whether or not she would be a superhero. I’m more interested in characters like Scarlett Johansson in Lucy. I’m less interested in people with superpowers because I can’t identify with them. Very rarely do they get killed off, and when they do get killed off, chances are they’re going to be back . . . somehow. Yes, I’d love to do a big splashy movie with a great female lead, but it has to be someone I can believe in.
With Scarlett Johansson and Black Widow then brought up:
I would love to do a Black Widow movie. That’s perfect, I would love to do that. That character is really interesting, she doesn’t have any superpowers, she just has extraordinary skills, and the world that she comes from, being this ex-K.G.B. assassin, I find that really fascinating, yeah.
While Marvel has yet to announce a Black Widow movie, a bonus for fans is that Black Widow will have a major role in the upcoming The Avengers 2.
It also can't be overlooked that Scarlett Johansson's recent standalone superpowered movie, Lucy, did pretty well at the box office with over $200 million.
It's unknown if a Black Widow movie will ever get made, but Marvel does have a batch of untitled movies coming out. Maybe Black Widow is among them:
Avengers: Age Of Ultron: May 1, 2015
Ant-Man: July 17, 2015
Captain America 3: May 6, 2016
Untitled Film: July 8, 2016
Untitled Film: May 5, 2017
Guardians of the Galaxy 2: July 28, 2017
Untitled Film: November 3, 2017
Untitled Film: May 8, 2018 (just announced)
Untitled Film: July 6, 2018
Untitled Film: November 2, 2018
Untitled Film: May 3, 2019