The first of these is Chromie, a gnome who functions as a "long-range artillery mage." We don't yet have a full rundown on her powers, but Blizzard says she has "the ability to bend time to her will" and "some of the highest kill-potential in the game." She'll be available starting on May 17.
The other new character is Medivh, a ranged Specialist coming on June 14. Blizzard teases that he wields "arcane powers to manipulate the battlefield to the benefit of his team -- and to the detriment of his enemies." He can transform into an invulnerable raven to move around the map safely and, more intriguingly, create a portal that his team can move through.
You can see Medivh in action in the gif above, as demonstrated in the trailer below that offers a quick look at Chromie and Medivh. The portal seems to be limited to a fairly short range, but the move is instantaneous and can be used to avoid walking all the way around obstacles in the environment. That's huge.
The video also reveals some upcoming skins, including one that makes Nova look like Overwatch's sniper character, Widowmaker. That probably doesn't bode well for the chances of seeing Widowmaker as a HotS character any time soon.