Blizzard just trademarked "The Dark Below" and it definitely looks like it has something to do with a new game or expansion!
- Computer game software; Computer game software downloadable from a global computer network; Downloadable computer game software via a global computer network and wireless devices; Downloadable electronic games via the internet and wireless devices
It could be anything, but it could also be the next WoW expansion. With Gamescom coming up in a couple of days, we could also be looking at the name of the next Diablo 3 expansion.
In any case, it looks like it's time to jump on the speculation train until we can get our hands on more information.