Blizzard Entertainment Senior Artist Dan John Cox has recently posted an announcement on Twitter. The company is looking to hire people on an "really really cool unannounced project" and he ensures it's "the best project and team he has worked with in his career".
While there aren't much details about the game itself, one thing seems likely: it will be a multiplayer FPS. The team is hiring a Senior Systems Designer and one of the roles' requisites is to have "extensive experience with games, including action, FPS, and online titles". Moreover, the Mid-Level or Associate Combat Designer candidate should have "broad, extensive player experience with FPS and action games, both single-player and online.
Here's Dan's tweet.