Currently in the tease phase, there's a leaked video showing new heroes Chromie and Medivh, as well as alternate skins for them along with Nova and Tyrande, and some new mounts.
Chromie, aka Chronormu, is a Bronze Dragon who is also the best dragon character in all of Warcraft. As a Bronze Dragon, she travels through time dealing with chronological anomalies and enlists the aid of Azeroth's heroes. She almost invariably does this in the form of a diminutive female gnome, and so it appears that for the most part, in Heroes of the Storm, she will use this Gnome form.
Medivh is a hugely important Warcraft character. The son of Aegwynn, Medivh gained his incredible magic powers not from the Council of Tirisfal - which is customary for Guardians - but instead directly from his mother. Unfortunately, that's not all he got. Aegwynn was possessed by the spirit of the Dark Titan Sargeras, who transferred himself to Medivh when he was in the womb. Sargeras would use him to open the Dark Portal, unleashing the Orcish Horde on Azeroth. Medivh was eventually killed by his friends Lothar and Khadgar, but he later returned as a spirit - presumably free of Sargeras' corruption - as a prophet to organize the heroes of Azeroth to fight off the Burning Legion during the Third War.
Chromie will get a fel-corrupted skin in addition to her master skin, with a fel version of Billie the goat to complement it. Medivh will get Knight Owl Medivh as an alternate skin, which has a kind of futuristic-knight look to it (maybe giving him an Overwatch alternate persona?)
Tyrande will get a similar futuristic-armor skin while Nova will get a skin that basically turns her into the Overwatch character Widowmaker.
Additionally, there will be two new mounts, a Elemental Wolf and an Epic Elemental Wolf.