I like Johansson's Black Widow in that she has really good chemistry with Captain America (the opening scene in Cap 2 with her giving Cap grief about his dating life was fantastic for me). The only issue is that I suspect doing a Black Widow movie is hard with her because she is such a big star before being cast in Marvel movies; too expensive (and we all know how cheap the head of Marvel is) I imagine. (Also, she probably has a full schedule at all times).
I don't think they can do Katee because she's tied to a couple of TV shows. (Longmire?). So many of the major casting choices we would love end up getting kibosh'd due to rather dumb things.

I wouldn't be surprised if Marvel has been holding off on announcing new movies / even changing their plans on movies because they haven't been able to get the people they want.