Blizzard has revealed that the next Heroes of the Storm hero will be Kael'thas Sunstrider, the Sun King, Lord of the Blood Elves, who led his people after the fall of Quel'Thalas before betraying the Alliance by secretly colluding with the Burning Legion. Friends presumably just call him Kael.
Sorry if that's a spoiler, by the way, but the Wow Wiki says that all of the above happened during Warcraft 3 and The Burning Crusade expansion to World of Warcraft, and given their vintage—just over and just under ten years, respectively—I think the statute of spoiler limitations has passed.
Anyway, I mention it all because there's really not a whole lot else to say about it right now. Kael'thas' arrival was confirmed on the official Heroes of the Storm Twitter account, but more information won't be revealed until 2:30 pm on May 12, during a Reddit Q&A with Hero designers John Hodgson and Kent-Erik Hagman.
Heroes of the Storm is currently in closed beta, but won't be for much longer. Blizzard announced in April that the open beta would go live on May 20, with the full launch following two weeks later, on June 2.