to begin my ideas of 4th specs i'd say 2 things:
1) new ranged weapon classification: bow and rifle are 2 handed // crossbow and guns are 1 handed ambidextry
2) new role: support: hybrid half-heal half-dps ( think of discipline or ex fistweaver) provinding buffs
now my ides:
DK: necromancer:: ranged magical necromancy spec
Warrior: soldier: ranged heavy soldier using heavy weapons as rifle, flamethrower, grenades, nets…
Paladin: purifier: melee support: using flames and 1 handed weapon in melee to purify his foes while Healing his allies
Hunter: Madness : 1 handd ambidextry ranged crossbow, sacrifice his pet to become crazy, with fast attack, blood rage system, and bleedings
Shaman: Earthwarden, tank, obvioulsy..
Monk: fistweaver : melee support, making mist and fist weaving 2 totaly viable but différent specs
rogue: ranged with 2 guns ambidextry, using stealh and combo points as a ranged spec could be really cool
DH: ranged with 2 cross bow ambidextry, using both fel magic and fel arrows
priest: inquisitor: magical ranged dps holynucker
displine is now a ranged support spec
mage: battle mage: melee fighter with staff, using both fire, ice and arcane in melee, with inovative system for combo to make interesting and new gameplay ( like 3 runes to use to cast a spell, 3 fire = 1 skill / 2 arcane 1 frost = another / fire + frost + frost = another …)
warlock: corruptor: ranged hybrid using dark magic to damage his foes and transfer life to allies