Varian Wrynn and Ragnaros are coming to HoTS, each bringing skills that set them apart from everything else in the roster. Varian Wrynn is capable of switching classes on-the-fly, while Ragnaros can transform into a giant, player-controlled Raid boss.
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Varian Wrynn can be a shield-bearing warrior, or a dual-sword-wielding sneaky assassin. In fact, Blizzard say there are six ways to play him, making the hero one of the most unpredictable opponents you can be unfortunate enough to face off with.
Ragnaros, meanwhile, can destroy entire lanes with rivers of fire, wiping out mobs with his many fire powers. His real interesting ability, Molten Core, allows him to take over a fort and essentially become a giant Raid boss, defending against an enemy siege or bolstering an attack during a push. This can be used on towers whether they're destroyed or not.
Varian Wrynn will be live on the test server next week, his full launch coming on November 15. Ragnaros is coming some time in December. Both are playable at BlizzCon. Here's a video of them in action.