Scarlett Johansson said this about her contract with Marvel:
"My contract has changed. Marvel and myself didn’t really know how the fans were gonna react to Black Widow the first time and how she was gonna be—they couldn’t anticipate how they would want to use her or if they would want to use her.
Right now I think this character is used well in this part of the universe, but I think that Kevin (head of Marvel movies) —I mean, we’ve talked about it and we both share similar vision for what could be a standalone series."
Sounds a bit like some stand alone movies could be on their way!