Think about it... DK, DH, Rogue, Warlock, Shadow Priest - they're all rooted in deep darkness - fel / legion, scourge / undead and void.
There's plenty currently to balance this out too, almost to where the 'good' classes are counterpoint to the evil - Paladin, Druid, Monk, Mage and I suppose Holy/Disc Priest.
So what's left? Warrior, Hunter, Shaman are all left somewhere in the middle - notably all much more strongly identified with horde than ally.
What would fill the void? Would it be good to have something down to earth rather than yet another super nautral?