It looks as though the next Hearthstone expansion has been outed by the Chinese Hearthstone website. Netease, the folks who run the site, uploaded a bunch of assets to the server and the assets were uncovered by crafty Hearthstone players on Reddit. The site has already removed the assets from the server.
The assets point to a new expansion or patch, called Knights of the Frozen Throne in China (the name might be different in the West), which essentially tells us that we are going to be visiting the Lich King. In the main banner for the site, you can see Heroes turned to Death Knights (those blue flowing eyes), as well as the top of the Icecrown Citadel (commonly referred to as ICC in World of Warcraft), where the Lich King lives.
Frostmourne is also in the image, though it's unclear how that will play out in Hearthstone. In World of Warcraft, the wielder of Frostmourne becomes the new Lich King.
Blizzard has not yet revealed this expansion, but it's likely that we will be seeing plenty of new items/cards.
