The mysterious fourth button added to the Hearthstone menu a couple days ago has been revealed as Tavern Brawl, a new mode that pits friends and strangers against each another in matches with unique, and sometimes unusual, rules that will change every week.
"One week might feature preset decks, while another week could have you crafting a brand new Tavern Brawl deck following specific guidelines, while others might offer buffs to specific minion types," Blizzard explained. Brawls can be started with random strangers, but there will also be an option to challenge players in your friends list as well. The fights will be free to join, but there will be a breaks of a couple days between each weekly event; Brawls will run from Wednesday to Sunday, with each week's rules announced on the Hearthstone Twitter feed and Facebook page. To celebrate the launch of Tavern Brawls, players will be given a free card pack when they win their first game of the week during the launch period, although specifically how long that period will be has yet to be determined.
Blizzard also teased two new heroes, for the Hunter and Mage classes, who will join Magni Bronzebeard as alternates for Rexxar and Jaina Proudmoore. These new heroes, like Magni, will feature a custom portrait, a unique card back, personalized emotes, and unique animations, and be purchasable from the in-game shop for $10 each. Players will also finally be able to customize their decks with selectable card backs and deck-specific heroes, "so if a new Hero has joined your collection, you don't have to leave an old favorite out in the cold."
"Your oozy, Naxxramas-minion-heavy Warlock deck can now be preset to use the Heroic Naxx card back, while your pristine Mage deck remains in Classic chic," Blizzard said. "Or you can just stick with the Pandaria card back for every single deck, because you like red and that’s how you roll."
On top of everything else, BlizzPro has also discovered that the Battle.net friends list limit has been doubled, from 100 to 200. There's been no official announcement of the expansion, but the site says it tested and confirmed the new ceiling.
The new Tavern Brawl mode, new heroes, and customizable decks will be added as part of the Hearthstone 2.7 patch, which is slated to go live sometime in mid-June. And since we're actually entering mid-June right now, I'd expect to see it happen very soon.