In my opinion there can't be an Old God expansion. It just doesn't make sense: in vanilla we had C'thun and the kingdom of Ahn'qiraj, also: the Twilight's Hammer and an Old God below Tirisfal Glades, discovered by the Forsaken. TBC? As far as I know, there was only a summoning of an Old God in Outland, by Arrakoa. WotLK - whole WotLK was filled with old ones, Ulduar, corruption, curse of flesh, Yogg-Saron, Titanic watchers... all "f*cked up" by an Old God/Old Gods. Cataclysm? Twilight's Hammer, N'zoth, Deathwing was Old Ones' puppet! In Mists of Pandaria we have Y'shaarj, the Sha and Garrosh. Everything connected with Old Gods.
So, why should they make an Old God expansion, when... every WoW expansion has some "old gods" in it? There can't be more old god-ish expansions like Cata, MoP or WotLK. But were they considered as "Old God expansions"? No.