Mage : Blood Mage. A healer spec
Rogue : A tank spec, possibly warden
Hunter : A healer Spec using salves and medical darts
Warlock : If they do not plan on a hero class Demon Hunter, then Demon hunter spec. If they do, then Apothethesis Tank spec
At this point, I believe thats that for fourth spec, as it would hybridize the 4 pures, so they can do something other than "dps", there fore killing time qs of lfr/lfd. But if they plan on 4th to all, heres the rest i think.
Warrior : Blademaster. 2h tank spec.
DearthKnight : Necromancer/lich. Plate int caster dps.
Priest : inquisitor dps. Burn them with holy light
Monk : Red Crane range dps
Shaman : Earth warden tank
Paladin : Heretic, range caster using shadow. Another int plate