Hmm. I don't think there is really much of a threat out there beyond the Old Gods, the Legion and the mad Titan that leads them (and maybe the threat of all the other Titans). Sargeras will be a very, very big problem whenever we finally have to deal with him. Such a big problem that it might not be feasible for the Azerothian encounter with him to occur in the medium of WoW. It might have to wait for WCIV because of how much bullshit deus ex machinae and faction cooperation we're gonna need in order to stand a chance against him. Plus, he's been hinted at having such a gargantuan size that having a 25-man (max) raid fighting him could just look laughable.
There is, of course, always the possibility that the Titans and other older races were themselves made by older, more powerful figures, but, like with the Titans and their terra-forming, whatever created the Azeroth universe, I don't think it gives a crap about the denizens of that universe. Or, if it does (possessing a Light or Shadow alignment or something (though, I think it's just raw, volatile and non-sentient arcane power myself)), it doesn't have the capability to change it all at once, like the Naaru.