Update: 12:06 PM EDT June 9: Added new art to the end of the post - tavern brawl art.
Update 1:36 PM EDT: According to the German Facebook page, there are no plans for co-op
New Main Button Being Added to Hearthstone
Thanks to a post on the Hearthstone facebook page, it looks like a new button is being added to the main screen in Hearthstone.
We are not yet sure what it will be for, although it is worth pointing out that the current buttons we have are all for various play modes.
We have confirmed that the new Heroes will be chosen via the collection manager, which means Zhang Dong's statement that there was still more to come appears to be accurate. We are indeed getting another large piece of content! What are you hoping for?

Clarifications
Quote from BlizzardThere are currently no plans for a co-op mode. (Hearthstone.de Facebook)
Will new hero portraits be usable in arena?
How does this selection process work? In the settings or when you click 'play ranked'
It'll be a part of the collection manager. - Zeriyah (Source)
Tavern Brawl Art
Some new art has been posted on the @PlayHearthstone Twitter account. We aren't sure if it has any relation to the new button, but it is worth mentioning.

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Posted Aug 29, 2017 (Frozen Throne)lol this was fun to discover it is obvious in retrospect
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Posted Jun 10, 2015 (Blackrock Launch)"Hentai Mode"
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Posted Jun 10, 2015 (Blackrock Launch)You could just use the nude mod, you know.
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Posted Jun 10, 2015 (Blackrock Launch)well if they wanted to have FUN in this game it be random modes like gain 2 mana or like all the adventer bosses are but it's blizzard and you know how much they hate fun in a game of theirs.
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Posted Jun 10, 2015 (Blackrock Launch)12 year old detected
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Posted Jun 9, 2015 (Blackrock Launch)1x1x1x1 mode
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Posted Jun 9, 2015 (Blackrock Launch)It's kind of funny how the mental health of a lot of Hearthstone fans hangs in the balance of a button not turning out to be shitty.
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Posted Jun 10, 2015 (Blackrock Launch)Yep! All it takes is one Button. One Button and everybody looses their mind!
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Posted Jun 10, 2015 (Blackrock Launch)DON'T TOUCH IT!!
IT'S THE HISTORY ERASER BUTTON YOU FOOL
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Posted Jun 9, 2015 (Blackrock Launch)People are going to lose their minds if this is just a button to access the new skins
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Posted Jun 9, 2015 (Blackrock Launch)This post already says it won't be a button for the new skins.
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Posted Jun 10, 2015 (Blackrock Launch)People have already lost their minds if they think this is even remotely a possibility.
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Posted Jun 9, 2015 (Blackrock Launch)Anyway, let's all hope it has to do with our prayers...
THIS
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Posted Jun 9, 2015 (Blackrock Launch)What if the button is something like that?
"Random $$$ cost for random reward" - 'RNG is fun' Ben Brode
:P
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Posted Jun 10, 2015 (Blackrock Launch)Your account has been charged for $99.95. Enjoy your golden annoy-o-tron! "Hello, hello, hello, HELLO!"
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Posted Jun 9, 2015 (Blackrock Launch)I suppose main hint is that a somebody on the picture is hiding in the barrel! This is certainly button for "Hidden mode"! Or maybe anyone have a clue what a drawing placed on the left? %)
Seriously - I just hope they won't do each built-in tournaments participation for another 9,99 $omething.....
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Posted Jun 9, 2015 (Blackrock Launch)2 main things from the drawing on the left... Green box kinda looks like the GvG poster, red is the innkeeper I guess.
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Posted Jun 10, 2015 (Blackrock Launch)Oh man, Iwasn't that serious! :)
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Posted Jun 9, 2015 (Blackrock Launch)slow day by the hearth.. think they just wanted to make us talk about something else then 10$ skins
either that or my refresh button gave up working on hearthpwn due to stress
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Posted Jun 9, 2015 (Blackrock Launch)its seems tournament mode, hopefully with the same mechanic of arena, with gold and packs as rewards.