Did the writer's expressly say Sal is gone for good? If so, I called that shit in the last season's thread. This show does seem to have a lot of Soprano's in it's blood, in that it's very willing to depart with characters without much melodrama or sense of outright closure. I respect that, because it's jarring and I for one like jarring drama.
I also can't imagine them really bringing in a black member of the firm and having it work out (in the universe of the show). I remember when last season ended that a lot of people were saying the show ought to open up the diversity of its cast, but that seems like it might a little false. The characters of this show are upperclass white people. Obviously the timeline means their going to deal with civil rights (as they've already shown), but I can't see Don or Roger really embracing the movement full on.
It would be kind of like having Tony Soprano bring a black guy into his crew. Both shows deal with insulated worlds going through dramatic upheavels, but neither of which are so open minded as to drop their prejudices.*
*(Not saying more color would be detrimental to the show, just that it doesn't seem like the route they'd go, given the themes).