From what I've seen in the BETA the most likely candidate for the Old God is NOT a Cthulhu Mythos creature. It seems to more fit a Dungeons and Dragons God named Tharizdun. He is a God chained by a coalition of deities to prevent the destruction of existence itself. He is the God of Eternal Darkness, Decay, Entropy, Malign Knowledge, Insanity, and Cold. You probably wonder, how is he like the Old Gods other than the fact that he was chained by multiple powerful creatures, well there is also something else you need to look at thats already in game. In a low level quest in Hyjal you manipulate the Ogres into leaving the area by using the Eye. During the short chain quest you read a book which says...
"In the beginning was shadow eternal.
Hate blazed forth, and FIRE was born.
Wounds scabbed, and so begat EARTH
Cries of anguish birthed howling WIND.
Wherein the skies wept seas of TEARS.
we live in the shadow,
the world we know
Built of rage, hurt, anguish and sorrow."
Tharizdun is the chained God of Eternal Darkness. He is also the God of Insanity and Malign Knowledge which are both things the other Old Gods are known for. If that wasnt enough he also has extremely close connections to the Primordials (Dungeons and Dragons version of the Elementals). He leads them under the alias of the Elemental Eye. He leads the Elemental Princes, who are empowered Primordials nearly as ancient as time itself, claiming to be the first Primordial. He also is known to manipulate Demons, and other life to his whim whether they are knowingly doing his will or not. Also, nearly all his followers are insane which can be related to then Twilight Cults. As well he also has an avatar whose name many people may recognize. The avatar's name is Shothragot. It is an avatar he made during the Twin Cataclysms which has laid dormant until its time of awakening in which it will look for the 333 shards of the key to his master's prison and free him. Tharizdun is also credited with creating the Abyss, Hell, and all the Demons who live there by corrupting a small porton of the Elemental Planes. Looking at that, wouldn't it make sense if it was he who created the Elemental Planes by doing the opposite of what he did in Dungeons and Dragons? Since the Old Gods are said to have created the Elemental Planes.