Update: His Spotlight is here!
After a series of teasers from Blizzard, the final image on their official Twitter shows Mal'ganis holding a minion. The attached quote is:
"I have been waiting for you. Are you ready to fulfill your purpose?" pic.twitter.com/zqftwyLNy4— Heroes of the Storm (@BlizzHeroes) October 7, 2018
If memory serves, this is a line Mal'ganis says to the Lich King (I think!), so I think it's fair to say he's odds-on for being the next Hero.
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