DKs didn't use Frost magic, but Arthas and the scourge did. It stands to reason that Arthas' generals would also be able to use it when you expand the concept. The cold of death, the Lich Kings connection to Northrend, and the Lich hero all lend themselves to that concept.
You expand the DH concept and you get Warlocks.
Sounds like wonderful mechanics to give to the Rogue class.Also, Heroes of the Storm opens up some new gameplay hooks to draw from. It doesn't have to be a 1:1 translation, but the Mark of Azzinoth combo system and high mobility dashing could be interesting in WoW.
There's a Tinker hero in HotS as well.