I'd say with WoW the biggest issue is that the caster main power fantasy niches are already covered. Monks were an RPG staple that worked to justify/expand Brewmasters into a workable class and Death Knights and Demon Hunters were two very Warcraft-specific power fantasies that just happened to me melee.
We've got our Gul'Dan-style caster in Warlocks, we've got our Khaadgar-style caster in Mages, we've got Priests, we've got Shamans and Druids. Necromancers would work but there's too much thematic crossover with Death Knights at this point. Just about every aspect of the established power cosmology and lore characters are covered in some niche.
About the only untapped angle I can think is some kind of rune-based caster. How you could work that in besides playable vrykul is beyond me.