Grom was designed for Heroes of the Storm, and honestly I wouldn't be surprised if he made an appearance sooner rather than later. In Warcraft III, Grom's first ability was Wind Walk, which allowed him to become invisible and move faster until he attacked. Mirror Image, on the other hand, allowed Grom to summon 3 illusions that did no damage, but it also allowed him to dispel any negative buffs on him. Grom's passive skill, Critical Strike, also granted him a 15% chance to do up to 4 times his base damage.
By far, Grom's most interesting skill is Bladestorm. His ultimate (or Heroic Ability for Heroes of the Storm's sake) made him completely invulnerable to magic while dealing great amounts of damage to enemies near him as he spun around in a tornado.
Since Grom recently returned to us in Warlords of Draenor, I think it would only make sense to add him to the Heroes of the Storm roster sometime in the near future.