If I were to design 4th specs this is what they'd look like:
Death Knight- A necromantic mana user that heals and steals health from minions, self, and enemies. Able to use int plate.
Paladin- A long range shockadin.
Priest- An anti shadow priest which would build upon holy fire and use more psychic abilities like telekinesis (damage dealing and knock back), empathy (healing and actual cc), and other astral abilities.
Warlock- A tanking spec which turns the user into a demon, reverting demonology into a pet spec again.
Warrior- A lancer spec using pole-arms and other long range abilities like a spear throw.
Rogue- An evasion tank that uses a shield and other self healing abilities.
Mage- A time wielding mage that heals, reverses time to undo damage, or enhancing the speed of damage on an enemy.
Shaman- An earth using, shield using tank, further representing the elements.
Druid- Already has a 4th.
Monk- A long range monk that uses more chi and lightning like abilities, which evoke the crane.
Hunter- A shadow hunter that uses magic and shadow based abilities. Maybe even infusing shadow into their pets.
The addition of these adds 3 tanks, 2 healers, 3 long range dps and 1 close range. This would bring the count up to 8 tanks, 8 healers, 11 lr dps, and 10 cr dps.
Instead of creating an entirely different class, refurbishing and building upon the existing ones would make it feel as if we have grown as classes.
In order to really set these off, I think that heroes of these new specs should come forward and further solidify them.
Alleria- Shadow Hunter
Illidan- Tanking Warlock
Thrall- Earth Warder
Velen- Light Priest
If you not this route, the new counsel of Tirisfal with Me'dan at the forefront might actually fit well, since it has representations of almost each class. This could introduce Me'dan as a potential bringer of peace along with Anduin to bring the Alliance and Horde together.