Here is a general idea for 4th Specializations:
Death Knight: Lich
Summary: The lich is a necromancer who raises minions from corpses. Casts shadow magic and frost magic.
Role: DPS
Primary stat: Intellect
Unique features: Raise dead.
Druid: Guardian
Currently exists
Hunter: Warden
Summary: The Warden is a vigilant stalker. Relies on stealth and patience. Has ranged physical damage attacks.
Role: DPS
Primary stat: Agility
Unique features: Stealth. Snare effects. Poisons.
Mage:Bronze
Summary: Bronze specialized mages are trained in the art of time. They specialize in time reversal, fast forward, slow and teleportation.
Role: DPS
Primary stat: Intellect
Unique features: Mobility. Speed. Stun/slow effects.
Monk: Ninja
Summary: Monk Ninja's are covert agents of deception, infiltration, and spying. They utilize stealth, bombs, and mobility to disable their foes.
Role: DPS
Primary stat: Agility
Unique features: Can use 2h swords/axes.
Paladin: Spellbreaker (aka Shockadin)
Summary: The Spellbreaker is justicar of all things magic. Relies on neutralizing threats and foes by using the Light to shock, stun, and steal magic.
Role: DPS
Primary stat: Intellect
Unique features: Stun. Steal magic effects.
Priest: Inquisitor
Summary: Iniquistors are passionate followers of the Light. As such, they seek out heretics of the Light and aim to destroy them in righteous fire.
Role: DPS
Primary stat: Intellect
Unique features: Can equip shields. Uses fire damage spells.
Rogue: Bard
Summary: Bards use deception, seduction, illusion, and persuasion to gain advantage in any situation. They are skilled in the arts of evasion and mitigation.
Role: Tank
Primary stat: Agility
Unique features: Can equip shields. Melee Tank abilities - incl. Active mitigation.
Shaman: Voodoo (aka Shadow Hunters from WC3)
Summary: Voodoo Shaman draw their energy from the spirit world. Utilizes heals and curses. Also versatile with melee weapons.
Role: Healer
Primary stat: Intellect
Unique features: Can equip 2h weapons.
Warlock: Demon Hunter
Summary: Demon Hunters are warlocks who are sworn to pursue and eradicate the existence of all known demons. They themselves have embraced demonology to gain insight to the behaviors of their prey.
Role: DPS
Primary stat: Strength
Unique features: Dual wielding 1h weapons. Metamorphism (a Melee combat version).
Warrior: Gladiator
Currently exists
This list is just a brain storm. Feel free to C&C constructively.