I think the time to do a Black Widow movie was pre-Avengers or at least pre-Agents of Shield. I'm sure they could come up with a story to tell and maybe even make a good movie to tell it in, but sandwiched between Thor 3, Guardians of the Galaxy 2 and Doctor Strange (or whatever) I suspect a Black Widow (with her loyal sidekick Hawkeye?) black ops spy thriller would feel a bit trifling. They could have used her in the lead of an Agents of Shield film, but with the show already filling that niche they've somewhat boxed her out.
Marvel has touched many genres in the movies. Iron Man was a techno-thriller, Thor was fantasy, and Captain America was a war-movie turned conspiracy-flick with Avenger uniting those genres. Then, though they all were science fiction, Guardians of the Galaxy took the lead in that department, with Ant-Man and Doctor Strange (with a bit of fantasy) right behind him. I'm hoping they shoot Black Panther in Africa as a political-thriller and not turn him into 'Captain America: The Winter Solder - Part 2 also in America with a Black Guy' but we'll see.
An Agents of Shield movie is exactly what they need to fill the espionage ticket. CA:TWS was close, but Steve Rogers as A Normal Guy doesn't count as con and disguise, and although Black Widow did, she wasn't the focus. If they aren't up for a Black Widow movie, AoS is the best route for that type of movie.
The obvious story to tell is how Black Widow went from bad to good, set in an earlier time.
I think a movie focused on Scarlett Johansson being a bad girl is a terrible idea.![]()
If it were up to me, I'd make Black Widow a Russian espionage movie where she's cleaning up her past bad deeds. Throw the Red Guardian in there with The Abomination (maybe, after Shield looses him in their fall) or Punisher's The Russian (without tits, saving that for the TV series), and it's a sweet movie.