Stan Lee attended Fan Expo in Canada where he mentioned that a Black Panther movie is happening.
While discussing if there will ever be a Black Widow movie, Lee stated more than likely she will eventually get one - as they all will - and confirmed Black Panther (via CBR).
“We haven’t made a Black Widow movie but she’s been in some of our movies, and she will be featured more prominently as we move forward. And 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, Dr. Strange and the Black Panther and there are others I am not allowed to talk about.”
Stan Lee has previously mentioned Black Panther a number of times, and he has also mentioned Inhumans as well.
To date, nothing official has been announced regarding Black Panther, but recently, Marvel Studios president and producer Kevin Feige did state that Black Panther and Ms. Marvel were the two most requested characters.
With Joss Whedon filming parts of The Avengers 2 in Africa, it's speculated Black Panther might make an appearance. It makes sense as Ultron might be after the fictious metal vibranium, which is found in Black Panther's home of Wakanda.
Recently, it's been suggested that Chadwick Boseman may be in talks for Black Panther.