And if by "anyone," you mean the denizens of the Blizzard forums, they all seem to think it's Nathanos Blightcaller, a Forsaken who has been sitting in the Eastern Plaguelands since the game began. He's recently been mentioned again in the game, as Varian Wrynn has sent Alliance players after him in Bolvar's place, and so it makes sense that he may rejoin the Banshee Queen and take his place at her side. Balnazzar, Varimathras' demon brother, is another guess, though he's currently serving secretly with the Scarlet Crusade, so who knows how he'd come to that position. And of course, besides Bornakk, anytime you hear B you have to think of our friend Bolvar, whose fate we'll probably learn in patch 3.3 no matter what. As for who it really is, we won't know for sure until we see them in the Undercity.