I don't think that's a given, though technically he has already played a Marvel character, in Men In Black. But as for the superheroics, he has a possible franchise with Hancock (which I didn't like by the way), so why would he need another one, and one where he might have less creative input? He's proven that he can launch a completely original hero without an established fanbase or publishing history.
Though personally I love for someone of his stature to take on an established comic hero because it might get folks interested in that character in print, it would get toys of that character made, video games, perhaps an animated cartoon, or a higher profile in a Marvel cartoon, and ultimately it would be a good thing for diversity in comics. If it is successful it might spark interest, or prove that black and other non-white characters, are bankable. The idea of black, white, and other kids playing with a black superhero toy or wanting to be that hero would be a good thing.
Roles I wouldn't mind seeing Smith take on:
Blue Marvel (The best role for Smith IMO; it's similar to Hancock in terms of power set, but the character is much more dignified, and since he's relatively new and unknown, there's a lot of leeway in how to interpret him).
Black Panther (not my favorite choice, but I know that with him behind it the movie would probably get made).
War Machine (in a spin-off franchise; I don't see Smith playing second fiddle to RDJ; that's why I can't see him playing Falcon; why the heck would he be a sidekick to Chris Evans?).
Isaiah Bradley (film would never get made, but I think Smith could get the seriousness and do the action required of the role)
Deathlok (this one sort of goes back to his sci-fi strengths, but can provide for some kickass action)
Nighthawk-Supreme Power (I would love to see this guy onscreen, but he's too hard core, too political, for the mainstream).