Some of these Heroes were datamined quite some time ago lke you said. Aside from that I think it was the Blizzcon video that showed the butcher, sylvanas (she's in now, though), rexxar and I think leoric and zuljin (maybe some more)? Can't remember too clearly. They were shown at the end of the video in the spider map background alongside all existing heroes. So they might be the ones that'll be coming up next.