Vanilla - ---- 6 ranged (DrHMaPSWk) 5 melee (DrPSRWa)
BC - 0 new classes
Wrath - 1 new melee class 6 ranged 6 melee (+Dk)
Cata - 0 new classes
MOP - 1 new melee class 6 ranged 7 melee (+Mo)
I'd expect no new classes for the next expansion, but if one would come, I'd expect a raged/caster one.