Bashiok -- Intellect plate
The situation with intellect plate is an ongoing discussion among the designers, but we have yet to come up with a solution that's better than just continuing to make intellect plate. ... Maybe someday there will be a new class that also uses Intellect plate."
Two words: Battle Mage
Druids will have Feral-Guardian as a fourth spec in 5.0, so there will be precedent within game mechanics. There is more than enough precedent in lore. There is also more than enough gear to go around. Besides, Azeroth really needs more shield tanks, and this is just the way to do it.
What other fourth specs would be nice? Just spit-balling here.
- Death Knight Unholy-Spellcaster: Ever play the RTS games? This is what they used to be.
- Hunter Beastmaster-Guardian: Put those polearms to good use and bring back Aspect of the Beast!
- Monk: Pass.
- Paladin Holy-Spellcaster: By any other name, a Shockadin
- Priest Discipline-Spellcaster: Spam Holy Fire and Smite, so basically a Holy Mage.
- Rogue Combat-Guardian: Just use Recuperate.
- Shaman Enhancement-Guardian: They have a taunt and a damage reducing, threat increasing weapon imbue. They have instant, no cost self-heals procced from melee swings. They can use shields. They can reforge for whatever dodge and parry they need, just as Druids do. They have Ghost Wolves that heal them. All they really need now is 6% less chance to be crit, maybe one more defensive cooldown, and the ability to click Tank on the LFD/LFR queue.
- Warlock Demonology-Guardian: Permanent Metamorphosis, I guess? I don't really know...
- Warrior Protection-Medic: Bandages! We've got to use up all that intellect plate somehow!