Healing
Tanking
Physical Melee DPS
Physical Ranged DPS
Caster DPS
Hybrid Physical/Caster Melee DPS
Those are the archetypes in WoW at the moment. The only one I see missing (simply based on combinatorics) is Hybrid Physical/Caster Ranged DPS. I don't even know how you'd implement that. I can only imagine something like Link shooting Light Arrows.
It will be interesting to see how they weave a new hero class into the game. With DKs it was easy, because for all of Vanilla and BC, Arthas was hanging out up north, building his army, and when we decided to move against him, the Death Knights were already there. Lore-wise, they always existed (since WC3 in their modern implementation) and had no racial discriminations because we're all equal in death or whatever.
But now, it's safe to say that the world is now more or less fully explored. There may be some islands or underwater or underground civilizations we haven't encountered yet, but it's very unlikely. There no more lore justification for a new class to suddenly show up.
That being said, it would take a lot more than a new, unbalanced class to breathe life in to WoW.