You would think Marvel would be exploring all potential avenues to sequelize that movie, including spinning off Black Widow into her own movie, especially since Iron Man 3 was such a big hit.
noted, Marvel Studios can only really do one or two movies per year, especially now that they've got the SHIELD series also. Black Widow will be in Cap 2
, and I imagine Whedon will want her back in Avengers 2
. In other words, her dance card is full for a while.