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.