Blizzard’s big Hearthstone announcement is an expansion called The Grand Tournament. It was revealed on a livestream earlier this afternoon, and it’s got a trailer (that you can view below the cut) and a dedicated website.
The Grand Tournament boasts a new board and new spell cards as well as cards featuring mounted characters, according to Blizzard Watch. In story terms, The Grand Tournament is a continuation of the Argent Tournament.
Source: The Grand Tournament