A new alternate class hero for Hearthstone has been announced. It's Lady Liadrin, the Blood Knight Matriarch.
Getting Liadrin is very easy if you already play World of Warcraft. If you have a character of level 20 or higher, you'll have her. Luckily, if you don't play WoW, you can still play for free up to level 20, so if you're desperate to have this character (though what do you have against Uther?) you can play on a free account. (EDIT: Actually, regardless of your previous play history, you will have to level a new character to 20. However, if you have a character at any level below 20, that will still count - I did it on a Worgen Hunter who started at level 14 last night.)
For some background: much as Uther was the founder of the Knights of the Silver Hand (the Paladin order that Human and Dwarf paladins belong to in WoW, and the Class Order all Paladin players will take charge of in Legion,) Liadrin was the first of the Blood Knights.
The Blood Knights have something of a problematic background. When Kael'thas made his alliance with Illidan, he captured a Naaru - a being of pure light - named M'uru and brought it to Silvermoon City. Whereas other paladin orders attained their Light-based powers through prayer and faith, the Blood Knights siphoned the power directly off of M'uru. It was a morally repugnant thing to do, but the Blood Knights felt they needed the power to hold off the Scourge and assert themselves after the devastation their society had faced.
When Kael'thas was unmasked as having gone over to the Burning Legion, his loyalists stormed Silvermoon and took M'uru. Liadrin, realizing now how wrong they had been, pledged her order to the true embodiment of the Light's principles.
While I'd personally have liked to see a Draenei like Maraad or Yrel as the new Paladin hero, it's probably best to give a Horde option. They've done this for Hunters (Alleria being an Alliance option) and Warriors (Magni, likewise.) Mages have Medivh as an alternate, though he's arguably unaligned. Liadrin's definitely one of the most important non-human Paladins, so it's probably the right choice.
No word on whether her Silver Hand Recruits will look different (maybe they'll be Blood Knight recruits?)