The first one being Cataclysm.
So, there's two old gods we havn't physically encountered yet on (Or should I say, in) Azeroth, jailed beneath the surface, one of theses two is N'Zoth, the other one remains unknown.
I always thinked N'Zoth should have been Cataclysm's last boss because he and his minions got so many more attention than Deathwing during the expansion (I'm aware Deathwing is one of N'Zoth's minions), I still think a whole expansion will be dedicated to N'Zoth with the Queen Azshara as a mid tier expansion boss, but WoW gets old and we don't know how many expansion will get out in the future, and there's still a lot of vilains to deal with in the Warcraft universe.
Since the aspect lost their powers, I think the Emerald Dream got weakened, and that Ysera can't ensure it's protection any longer, so i'm now thinking we will have to fight N'Zoth's forces into an "Emerald Nightmare" raid since he seems to be responsible of all of this.
As for the fifth old gods who remains unknown - my guess is that we will never see him.
So do you think that we will see old gods again in WoW ?