Reading similar threads styled around giving characters specs revolving WCIII heroes and such, I couldn't really think of the point beyond coolness factor. Many of the abilities associated with everything from Wardens to Demon hunters have already been peppered through different classes and specs to the point where it would be nigh-impossible to create a viable fourth-spec with real flavour from what's left.
There is, however, another way to look at it: What about pure DPS classes wanting shorter queue times and/or different roles?
Easy way to solve this: Build a utility spec, either tanking or healing, into the existing pure DPS classes. Without further ado:
Take the glyph. Make it a spec. Give tanking locks purple glowing claws for their melee attacks. Come up with some cool active mitigation ability using the demon. Maybe even having the lock actively shunt 90% of incoming damage to the pet, and the healer having to focus on healing the pet. Just a thought.
I like the the concept of a Temporal Mage, reversing time in a localised field to wipe wounds from existence. Yes, I admit stole this idea from Bleach. It's still cool. Lore even fits with the new mortal guardians of the Caverns of Time being able to impart new magic to mages.
Sadly (well, not really sadly... We have it now and it's awesome) this idea has already been implemented in the form of a Brewmaster. I thought the idea of evasion tanking three years ago would be awesome for rogues. It could still work... Maybe some thematic variation to differentiate it from Brewmasters?
I love my idea for this. You probably won't. Tanking spec for hunters: Let the Hunter be the "pet" while the player drives tanking-themed pets. Perma-Eyes of the Beast on steroids with the hunter as a "minion".
A couple of others for hybrids, just for the heck of it:
Yes they can heal, but I honestly think they need a tanking spec as well. Something about channeling spirits to absorb damage and thumping things with giant totems. I admit this idea is partly stolen from elsewhere, but it's a damn-good idea.
Paladin: Ranged DPS.
Int plate. Shockadin. Wacky Hijinks Ensue.
Warrior: Ranged DPS
I had a cool concept that unfortunately doesn't really fit established WoW archetypes. A ranged-DPS spec for Warriors where a one-handed sword, mace or axe it held on a chain and thrown at enemies before being retracted.
Monk: Ranged DPS
Ok, this one's a bit of a stretch because monks are basically awesome and can do everything... But think about it: Alcohol is flammable. And portable. And can be weaponized.
Anyway, I think that would solve the 4-spec problem for everyone. Plus, with everyone Having a utility spec, there's be 10-odd new "classes" in the game, as well as nobody being able to viably complain about queues anymore. Give it some thought.