She's as least as "superhero" as Batman, maybe a little more with her stingers. They could even give her a Widow-mobile or jet, and other Bat-like gadgets.
It doesn't have to be a Black Widow movie, but she's already appeared in three popular films, played by a popular actress, so she is the most obvious choice. The star of a low-selling comic with a name more associated to two male heroes seems like a riskier move.