Rucka would still probably be my first choice for a Black Widow book. Q&C was great, and he's done a lot of stellar superhero stuff at DC, too, he likes the character and it's very much in his wheelhouse. I also think Greg Rucka is a name that really appeals to the section of the fanbase that wants to read books about women, he's probably second only to Gail Simone in that regard.
And yeah, I think if they are skeptical about an ongoing, just doing a miniseries is a nice way to capitalize on the movie hype without much commitment and also a way to remind readers she does have a POV. I would be very happy to see a 616 continuity miniseries, especially if someone like Rucka wrote it. Unfortunately they seem to be cutting down on minis across the board, limiting them to event stuff.