A Black Widow movie would have to be a SHIELD led Avengers covert ops movie, hopefully co-starring Hawkeye, and quarterbacked by Fury homself. (w/ cameo's from AoS characters - like Mockingbird, Fitz, Coulson, etc etc)
I think they should do a series of shorts revolving around her and Hawkeye completing covert OPS missions inbetween all the movies, or even the off camera stuff they were doing during the movies.
I know tons of people would have killed for several 10 minutes Hawkguy Bro shorts before the movies or on the DVD's or even as a limited Netflix series.
and speaking of shorts, once IW is all done, I woudl love if they did a series of shorts on the DVD release that depicted what Thanos was up to since his 1st appearance in Avengers, and how he was orchestrating things that led to him finally just saying "**** it, I'll do it myself", that's of course assuming they dont' cover it in the 2 movies he will start in in 2018 & 2019.