Adding another class requires a TRAMENDOUS amount of balancing to work, especially in PvP.
Lets be honest, a new class will not bring back the sub numbers. The players who quit will not return long term becouse theres a new class, perhaps to try it out.
So I dont think Blizzard sees much profit in spending that much time and resources on something that will not benefit them much, if at all. 1 shop mount will probably bring more income to blizzard than a new class will do.
I dont think so atleast. If you do however believe we will see a new one, please share your thoughts below