There are stranger places to get Marvel casting news than a Reddit AMA, but this falls more into the speculation. During an interview with Reddit yesterday Idris Elba (Thor: The Dark World, Luther) was asked: How quickly can we get you signed on to play Luke Cage for the Netflix series?
His only response: “Uh, I think there’s an actor already signed on for it, fortunately or unfortunately.”
There is a Marvel policy against an actor playing more than one character in the cinematic universe so playing Heimdall may have prevented him from playing Cage, but that is just speculation. As is the idea that Luke Cage has in fact been cast… but from Elba’s comments you can infer that and with New York Comic Con coming up with a Daredevil TV series panel introducing the cast, maybe we’ll get some other casting news out of that for the three other series.