There's this guy that goes under the name Axolotl, who has been predicting Heroes of the Storm leaks with a nearly 100% accuracy since November 2015. We don't have much information on who the guy is, but he probably has an insider in Blizzard as his Hero leaks have been entirely accurate and he mentioned Chromie a half year before her release. You can see the history of his posts here. On November 7th, 2015 he posted the new Heroes that would be added were Greymane, Lunara and Cho'gall, specifically mentioning him as a two-player Hero. This happened at around 1:43 a.m., literally hours before the official HotS panel on Day 2 unveiled these Heroes. All three were correct. On the same day, he specified that the Diablo 2 necromancer was coming soon (Xul). He even predicted Li-Ming and on April 12, he not only wrote about Heroes but also mounts and skins that would be added to the Nexus. Fel Queen Chromie and Knight Owl Medivh were listed here and that's not all, he even wrote some of their talents.
His latest post in May unveils
Gul'dan -- confirmed being worked on with a release around the Warcraft movie.
Trait: His trait revolves around sacrificing life to deal extra damage then stealing life.
Abilities: Hand of Gul'dan, Chaotic Energy, Rain of Fire, Drain Live, Life Tap, Fear
Samuro (Warcraft III Blade Master with Stealth).
Abilities: Illusion Dancer, Burning Blade, Zanbatou, Wind Walk
Two new Starcraft maps will be added to Heroes of the Storm next. The first one's called Braxis Holdout the second one Warhead Junction.
New Hero Type
Blizzard is currently working on an entirely new Hero type. Apart from Warriors, Assassins, Supports & Specialists a fifth one will be added soon.
A new Volleyball Li-Ming Skin will be released in the future along with Mechalord Gazlowe.
Red Hare & Gyrocopter.
Heroes Being Worked On
This list contains Heroes that are being worked on, but that won't see a release anytime soon.
Shandris, Valerian, Varimathras, Ragnaros, Mengsk, Maiev, Imperius, Gromm, Garrosh, Brann, Baal, Alleria, Deckard Cain, Rocknroll Racer, Selendis, Stukov, Vashj, Tosh, Moira, Varian, Belial, Deathwing, Ghom, Cydaea, Zul'jin, Auriel, Zarya, Aidan.
1. Auriel is a Diablo III angel with a resurrect ally ability.
2. Varian is a typical WoW warrior build with charge, taunt and cleanse.
I'm really curious about the next in-development video that should be published soon.