IRRC, the casting of Sam L. Jackson was a comic book homage to an actor that got translated to the movies.
They even had the Ultimate Nick Fury character voice his desire to be played by Sam Jackson in the movie.
I doubt that Mark Millar has minority appeasement or even diversity on his mind when he thought up Ultimate Nick Fury .
Its more likely that the dude is a big Tarantino/SLJ fan.
Each major Marvel character has had his own movie to appear in before showing up in the Avengers. The only Black Avenger shown so far in the movies is War Machine. He could join in Avengers 2. Falcon might show up in Captain America 2(or you could show BP there).
If a Black Panther movie ever gets off the ground, he will end up in the Avengers franchise.