Your comment marks the essential problem with adding classes; the game doesn't have any niche roles for a new hybrid to fill.
Though many people in the WoW community were not around at the start, the idea of Hero classes in their original sense; Your character growing into the new class, is really the only way to go. Like Mages? Work toward turning them into an Archmage. Like Druids? Work toward turning them into an Archdruid. And so on.
Sadly the developers never bothered to allocate the resources needed to accomplish this initial goal.