Today we may know the name of the expansion: Mists of Pandaria. According to MMO Champion, Blizzard filed a trademark with this title on July 28th, which is exactly in line with how the company has procured titles for the previous expansions. The trademark specifies that this is for "computer game software."
The Pandarens are one of the more light-hearted races in the Warcraft franchise, with kung fu panda bears from a secret empire somewhere in Azeroth. Despite being fan favorites, the Pandarens have been rarely seen in WoW, while Blizzard has used them in a couple different April Fool's jokes in the past.
We'll be keeping our eyes on Blizzard for any official confirmation.