PC Gamer reported that they received a small scroll in the mail teasing the next Hearthstone expansion. The scroll, when opened, played a small fanfare and had the following message written on it:
Champions are gathering from around the world to witness the next exciting thing Hearthstone has in store. News travels far and wide in search of brave new faces ready to answer the call to arms — Are you one of them?

Muster for battle and mark July 22 on your calendar. The next bit of news won’t come by carrier pigeon — thankfully, we have much better ways of communication now. Victory awaits.
This teaser was very similar to the Goblins vs Gnomes teaser, where a blueprint of a demolisher was sent out to varying parties. This time however, the scroll and fanfare seem to hint at an Argent Tournament themed expansion, modeled after the Tournament added in patch 3.1 in World of Warcraft.

PC Gamer uploaded a picture of the scroll: