I have a wild guess:
Next expansion there will not be a new class, instead they are giving the fourth spec to every class:
Warlocks - Tank, based on the current experimentations with DA
Mages - Melee dps focusing on Lightning as element
Rogues - Healing spec, themed after Bard. Unique concept of instruments as exclusive spec equipment.
Paladins - Ranged dps spec, themed after Shockadins.
DK - Ranged dps spec, themed more directly at necromancy. Revolves into micro managing several minions at the same time.
Druids - Melee dps, themed after druids of the fang (a reason for them to finally invest on their lore): you spend most time dpsing in humanoid form but activates snake form at key times to apply powerful poisons.
Priests - Ranged dps, themed after exorcism (goes well with demon-themed expansion). Can bring more interesting mechanics to smite and holy fire.
Monks - Ranged dps that uses chi and a bow.
Hunters - Melee dps, hunters will be able to use melee weapons again on this spec, and those old skills like raptor strike will make a comeback. Plus marksman will be reworked to be able to exist petless if the owner wants, with a mechanic similar to grimoire of sacrifice, but only avaliable to MM.
Warrior - Ranged dps spec, with exclusive equipment called "chain-glaive", wich you craft with bladed weapons and a chain. It's a ranged class, but his main attack "glaive toss" has a cast time, so it's like a mage that deals ranged, physical dmg.
Shaman - Tank spec, themed after the earth element.