To combat this, Blizzard has announced the creation of a new game mode named Standard in which players can use only cards released in the past two years in addition to basic and classic cards. The current gameplay is being repackaged as Wild mode, which allows any cards to be used. The coming year will also see balance changes to basic cards, and players will get an extra 9 deck slots so that they can have separate Wild and Standard decks. Read on for a video from Lead Designer Ben Brode with an explanation of why this change is necessary for the future health of Hearthstone.
Source: Hearthstone Devblog