Hearthpwn recently translated an interview with Hearthstone Executive Producer Hamilton Chu originally published on Chinese website nga.178.com. Hearthstone's future is being discussed with the addition of new premium content.
Game content and features
They're working on new heroes like Varian instead of Garrosh for warrior decks. New card backs are also being considered and these are separate from the ones that are to be awarded during ranked season play (with even more rewards to be planned for the ranked ladder season). New game boards are also going to be added. There is no plan to adjust the length of seasons yet although the team is watching it carefully.
For the time being, Hearthstone will consist strictly of Warcraft themed elements. Anything not Warcraft related won't be added. The game will eventually feature achievements. Adventure modes will have new quests, new game stages, and add an additional 10 to 20 new cards. Expected release date will be some time after the game is official released on the PC and iPad.
Even though Hearthstone isn't out yet, the developers are looking ahead and planning the first expansion which will likely contain 100 to 200 new cards. Chu confirmed that Death Knights and Monk classes are not being worked on. 2v2 mode and observer modes are not actively being worked on (although observer mode is on the to-do list).