In the latest Patch 4.1 PTR Build, two new halloween masks textures have been added to the game files:
They seem to be early-alpha texture but according to what happened with the datamining/leak of the Cataclysm races 2 years ago, it looks like Pandarens will be the race of the next expansion! The name of texture for any piece of headset always ends with the first 2 character of the name of the race (Pa) and the gender (M/F), according to how the textures look it's really easy to conclude what the new race is.
More datamining reveals that the expansion is apparently named Panda Mania! and also uncovered a possible trailer ....