Polygon accidentally published their article on the next HS expansion, Ashes of Outland, early.
http://archive.is/2WqXT
https://webcache.googleusercontent.c...&ct=clnk&gl=us
The story of the expansion involves Mecha-Jaraxxus creating the Rusted Legion by reviving and mechanizing the fallen heroes and villains of Outland.
It will be introducing Demon Hunter as the first new class since launch and will also see the Priest class undergo a core redesign to solidify it as a control archetype class.
Illidan Stormrage will be the Demon Hunter hero, and will be introduced via a free story campaign that retells his origin story, which will also reward players with the class's core cards. The adventure will be made available on April 2, and the class will become playable in other modes on April 7.
Demon Hunter hero power is Demon Claws, 1-mana, gain +1 attack. New keyword exclusive to the class is Outcast, cards gain special effects when they're the right- or left-most card in your hand. Class is aggressive with a focus on attacking using your hero, AoE damage spells, RNG enemy damage effects, demon minions, minions with rush and bonus damage dealing effects, and card draw. Class is lacking card generation and hard removal.
Priest redesign will be altering their core cards significantly, with some cards changed and others replaced. Their OTK cards, including Velen, are going to be Hall of Famed, it seems.
New expansion mechanics include Prime minions, class Legendaries that get shuffled back into their owner's deck and become stronger when they're killed. Kargath Bladefist for Warrior and Lady Vashj for Shaman.
Another new mechanic is Imprisoned Demons. They are played on the board in a dormant state, during which time they can't be interacted with, and activate after two turns.
Kael'thas Sunstrider will be given out as a free neutral legendary minion tomorrow (March 17). The only detail on Kael'thas is that he makes the third spell you cast during your turn free.