Blizzard is overhauling Hearthstone, they are making some big changes to the core of the game. These changes include a fresh new format for the game as well as a much desired additional deck slots.
Hearthstone multiplayer games will now be separated into two modes. Standard, which requires players to use decks build from cards released in the current and previous calendar year, as well as basic/classic packs. As of this article that includes the expansions for Blackrock Mountain, The Grand Tournament, and League of Explorers.
This will also include Wild mode which is basically the current Hearthstone were cards from any pack can be used to play with others.
However, the big thing that most users will look forward to and be excited about is the change to doubling the deck slots. Meaning once you have unlocked all 9 heroes, you will be able to utilize 18 deck slots to play with and build.
These changes are being made to try and help balance the game for old and new players to be able to enjoy the game and be competitive.