Ernie Hudson would've been fantastic. The only downside to him that might've happened is that by sheer force of personality, he could have been just as forceful a presence as Jonah, and Robbie is supposed to be the mellower version, stern but more fatherly.
I think Bill Nunn was fine in the role. I like him as he's the only serious voice in the bugle scenes, and he plays it well.
I recall someone saying over in the 'black superheroes' thread that he didn't like that Bill Nunn wasn't physically like RR, and that there is less care taken on black characters on looking like their comic counterparts. I can see his point, but after a while i forgot he was more overweight than his comic counterpart, and you get that kind of thing elsewhere with white characters too, like Pat hingle in the Batman series. His weight bothered me too as Gordon, but then I forgot about that as it increasingly became apparent that they were never going to do the character anything like teh comic character, the weight wouldn't have bothered me afetr a while if he had been more like the character.
Ernie Hudson worked with JK Simmons in OZ. I suppose I'm more used to Ernie H being a really dominant personality, perhaps this would have risked some of Jonah's overbearing personality being muffled. ie You don't want the backing singer to be better looking/sounding than the band's lead singer, takes teh focus of attention in the wrong direction.