As I said, Helya was hinted at. She escapes and threatens all of Helheim will stand against us.
The shaman concept tier set is based off the elemental hidden skin which is Amani themed. Just like how all tier sets at launch are based off an artifact appearance, it's no more a hint at a future raid than the paladin tier set hinting at an Ashbringer raid. You may have a point with Vol'jin but it's something only Horde sees so it seems unlikely.