I'm going to give Marvel Studios the benefit of the doubt here. Let's take a look at everybody they've introduced via solo movies.
Iron Man, Hulk, Cap and Thor are off the table IMO. They're easily the four most high profile characters MS had at their disposal, so I don't think there was ever any doubt that they were coming out before Black Panther.
Guardians Of The Galaxy is considerably lower profile, but I can see why they put that out before Black Panther as well. Firstly, and most importantly, it blows the cosmic side of the Marvel universe wide open. Secondly, contrary to popular belief, a well made space/sci-fi adventure with a large scope is basically a safe bet. We're basically talking about the Star Wars, Star Treks and Avatars. A talking raccoon and walking tree? Um, that just made said bet even safer by really appealing to children. That isn't to say I think Black Panther isn't a safe bet... I believe it very much is.
Personally, I would have delayed Ant-Man in favor of a Black Panther movie, but like Feige himself said, the only reason they went forward with Ant-Man was that Marvel basically fell in love with the concept of an Edgar Wright Marvel movie way back when, and Edgar Wright wanted to do Ant-Man... or more specifically, To Steal An Ant-Man.
In any case, I think it's a very safe bet that the next three new solo film properties that Marvell introduces will be Black Panther, Captain Marvel and the Inhumans... in some order. I think all the hair-rending and hand-wringing over the lack of a Black Panther film is a little premature at this point.