I personally feel like it's too late for the Black Widow solo; we could have done with her origin much earlier. I don't see the draw of giving her a solo outing now - it's only because fan service desires it that it's happening in the first place.
Black Widow, especially the MCU Widow works in an ensemble setting as a support character. For me, she's never going to be solo material. Maybe she could have been once upon a time, but I'm not remotely getting that vibe for her character now - the same applies to Hawkeye, Coulson, Maria Hill and Nick Fury. I have no qualms about any of these characters appearing in prequel films, but I have little desire to see them in their own sequel.
For the most part, Marvel, and in turn, the MCU deals with superheroes more than it does spies and unpowered street-level vigilantes; the likes of DareDevil are an exception because of his enhanced senses.