At Fan Expo Canada Stan Lee responded to a question about a Black Widow movie:
The chances are she will have her own movie because eventually all the superheroes are going to have their own movies. They are already working on Ant-Man, Doctor Strange and the Black Panther, and there are others I am not allowed to talk about.
It does seem like he would know, but it's not been confirmed anywhere else. I really hope that this one turns out to be true.
(Also, wanted to add: Sorry if someone posted this, I tried searching for it and couldn't find anything. If this has been reported, feel free to just look at the cute lil' black panther pictures and go 'awww.')