TheInquirer
is reporting to have overheard some Blizzard employees chatting about an as yet unannounced project called "hydra".

Apparently the conversation between the two devs went a bit like this:

Blizzard Employee 1: What are you working on?
Blizzard Employee 2: Starcraft II, and you?
Blizzard Employee 1: I am working on Hydra.

Blizzard themselves are sticking to a strict "no comment" about what exactly they are working on outside of World of WarCraft, and StarCraft II, so it's worth taking this with a pinch of salt, but it seems probable enough. What could it be though? Diablo III is where all the bets are going currently.

We'll keep you updated.