Because everyone loves an announcement of an announcement, Blizzard has pre-announced that it will announce (I'm getting dizzy) the next Hearthstone expansion during the HCT Spring Championship tournament next Thursday, July 6 at 7pm Pacific.
We don't know what theme will follow the current expansion, Journey to Un'Goro, but there are a few rumors—the most prevalent of which is something to do with Warcraft's infamous Lich King and the icy region of Northrend. A lot of signs point that direction, but there have also been whispers of a potentially Dalaran-themed set.
We do know, however, that it will be another 'full' set, now that Blizzard is no longer doing Adventures, so we can expect around 130+ cards. Speaking of Adventures, perhaps we'll see some of the free single-player content which Blizzard mentioned would be coming to compensate for Adventures being phased out.
You'll be able to watch the announcement as it is announced (but not until it's announced, as this is just announcing it will be announced) on the official Hearthstone Twitch channel, along with the actual tournament. We'll have boots on the ground covering the event, and hoping not too many Pros have picked Quest Rogue.