I think a ranged weapon spec that feels like the Trooper from SWTOR or Roland or Axton from Borderlands 1/2 would be a great addition to WoW, and wouldn't really feel all like MM Hunters. If Fire Mages and Destro Warlocks can different enough, then so can that.
A grenadier type class would definitely be unique, and a tank spec in which you are literally in a tank is without equal.