It seems like it's the Burning Legion since they destroy whole worlds on a regular basis (despite failing with Azeroth) while the Old Gods have been contained to Azeroth from what we've seen. There is speculation and reason to believe other beings like Azeroth's Old Gods are out there, but we have no solid certainty on what they've done to other planets, etc.
Burning legion, Titans, and old gods are all kind of connected. The Titans are pretty significant threat though. as xanzul said they could just come back take a look at trade chat and go "Nope" flip a switch and wipe us all out.
It's us, we cry and cry endlessly over just about anything and when blizzard delivers, we change our minds and don't want it anymore.
You know, there are actually more than 5 people who post on these forums, right? Just because some people want something, doesn't mean everyone does. Strangely enough, what pleases some people might really annoy others - and those others can and will complain about it when it happens.