Hearthstone's new expansion is going to be 1920s gangland themed.
It seems that after the Cataclysm, the goblin town became a real city and of course, being a Goblin city, naturally there are plenty of bootleggers and protection rackets. There are three major gangs represented in the expansion. Each gang will have tri-class cards that can be used in any of the three classes associated with the gang.
Grimy Goons are a big protection racket who use crazy inventions like piranha guns and walk around in the open.
Jade Lotus are secretive assassins who seem to be mostly Pandaren in flavor.
Mage, Priest, Warrior: The Kabal are a secret society that uses illicit potions.
The Kabal leader legendary minion will allow you to create custom spells, choosing a cost and two effects (that are proportional to that cost.)