Realizing that introducing a new class into WoW is a pretty tricky and hard to balance thing, as seen by the introduction of the Death Knight in Wrath of the Lich King, Blizzard have taken a step down for the moment, and are instead introducing two new races to join the Alliance versus Horde conflict, similar to Burning Crusade.

With their island torn apart by an erupting volcano, the Goblins of Kezan, neutral till now, are forced to move from their homeland. Finding it harsh and profitless to deal with the Alliance, they soon find a new affiliate in the Horde. And after all, the cuteness factor bought by the Gnomes lacked a little, so this addition just made sense.