Black Widow and Doctor Strange movies are on the table, I think as possible Phase 3 movies. That doesn't mean they will happen of course. Right now they are in the 'rolling around, see if we can make them work/make a lot of money off of them'. I think that a lot of it will depend on how well Guardians of the Galaxy works out. As much as they love to talk about how the Avengers are supposed to be the 'lesser known characters of the Marvel Universe' compared to the X-Men, Spiderman or the Fantastic Four... they aren't. They really aren't. The Avengers as a group, Captain America, Iron Man, Thor and the Hulk are some of Marvel's biggest named characters.
Less popular Avengers like Black Widow, Hawkeye or any of the Captain Marvels would be riskier to have run with. Right now Guardians is in that big risk category for them. They've used their big name star power to build brand credit... Will that credit be enough to pack people in to see less well known characters? And will they produce another movie that's fun to watch on it's own, while at the same time adding more to the shared universe?
If we do get a Black Widow movie, I hope that it will be co-starring Hawkeye. Spy thriller with super heroes will work, so was shown by Winter Soldier. And they are shown to have worked well together in the past, and like each other. Also importantly to trust each other enough to work together post SHIELD and outside the Avengers. So it make sense.