Following the latest update for Hearthstone: Heroes of Warcraft, the possibility of a co-op mode for Blizzard's popular card game has arisen. According to Hearthstone site Hearthpwn, the game code hides some very curious references, which you can see below:
VO_HRW_TEST_00 - I will cleanse you both of the Curse of Flesh!
VO_HRW_TEST_01 - Well done! You've defeated the boss together.\n\nBoth players win!
VO_HRW_TEST_02 - Defeat Gearmaster Mechazod together to win!
VO_HRW_TEST_ABILITY_00 - Damaging a hero.
VO_HRW_TEST_ABILITY_01 - Destroying highest attack minion.
VO_HRW_TEST_ABILITY_02 - Throwing 3 bombs.
VO_HRW_TEST_ABILITY_03 - Damaging all other characters.
VO_HRW_TEST_ABILITY_04 - Gaining 2 attack.
VO_HRW_TEST_ABILITY_05 - Dealing damage to both heroes.
VO_HRW_TEST_ABILITY_06 - Silencing Lorewalker Cho. Permanently.
VO_HRW_TEST_ABILITY_98 - Target board full. Clearing target board.
These lines seem to mention a battle that pits two players against a "Boss", most likely controlled by the AI, similar to what we have seen in the Curse of Naxxramas and Blackrock Mountain adventures.
Based on this evidence, it seems very likely that Blizzard has looked at the possibility of a co-op mode, but it isn't clear if it's something that has been dropped, or is being planned for the future, be it in the new The Grand Tournament expansion or in a subsequent adventure.
As for The Grand Tournament itself, it will introduce 132 new cards when it lands on August 24.