Ok, this just occured to me because I realized how tall he is. But I think he's the best possibility I've thought about so far.
J. August Richards
And I'll give multiple reasons why it would be wise to cast him. First of all, he's 6'3". He can play tough and streetwise well, but not 2 dimensionally at all. White sci-fi/comic type fans, who a movie like this probably needs to bring in to be reasonably successful (in the bussiness sense) mostly already know him and appreciate his work on Angel and may be more likely to check it out on that basis, than if it were an actor of his class who gained fame in the realms of entertainment more black actors do (urban movies, tv, and also music).
In addition to the marketing standpoint, J. August is someone known for being unusually dedicated to projects and fans that he works on, and is himself a big scifi fan (his goal is to one day own a lifesize replica of the Millenium Falcon).
Anyway just thought I'd throw this idea out there. I think the role of Luke Cage is different enough from Gunn that it isn't really typecasting. One's obviously the lead in a movie and the others one of a few sidekicks on a serial drama. In addition to other things.
I also really think he's the right age for the role. Why so many suggest people almost or over 40 is beyond me. I think the story's only likely to work if it's a relatively young man who get's sent to jail.
Lastly I do think that him being not Luke Cage big is not a negative, it's likely that a role like this would require someone to start off at a physical place where they could make you appear average sized and end up jacked. So someone who is as dedicated as him to a role could feasably put on 35+ lbs of muscle