The resume of veteran voice actor Lani Minella has let slip that the next Hearthstone expansion will be themed around the Un'Goro Crater, which is Azeroth's equivalent of the Land of the Lost. The since-edited document, captured for posterity by Reddit, references Minella's work on something called Hearthstone: Lost Secrets of Un'Goro, with portrayals of Golakka Crawler, Pteradactyl, Anklesaur, Hydra, and Bronotsaurus.
The Un'Goro Crater, as the WoW Wiki makes clear, is not a place for the unprepared tourist. "Although the area is almost completely isolated from the rest of the world and sandwiched between three deserts, the region is filled with exotic plant life and even more exotic creatures," it says. "Ranging from peaceful earth elementals to raging devilsaurs, the locals are not to be trifled with."
Minella has previously voiced characters in Hearthstone and the Whispers of the Old Gods expansion, among many other games, which lends some credence to the leak. So does the fact that ungoro.com is a live url, although a whois indicates that it's actually been around for several years now. And yes, that's about it as far as supporting evidence currently goes. Thin, definitely.
But a new adventure is inevitable, and the Un'Goro Crater sounds like an interesting place to dig into—plus, given the widespread dissatisfaction with the current meta, the next Standard rotation can't happen soon enough. When the new set launches, we'll finally see Totem Golem and Tunnel Trogg dumped into Wild, which frankly can't happen soon enough. Meanwhile, at least word on balance changes are due before the end of the month, almost certainly aimed at toning down those dread Pirates.
I've emailed Blizzard about Un'Goro, and will update if and when something more official is forthcoming.