Come on, even if it comes as a DLC, there are already Hyperboreans on the game as mobs and npcs and morph items to temporary change the player to a hyperborean(one permanent too if i remember correctly), Hyperboreans IMHO should be able to be Necromancer(not HoX and Demonologist), Bear Shamans, Dark Templars, Assassins, Rangers and Guardians.
Why not HoX and demonologist?
Their magic focuses around the cold of their land and control of the dead. Hyperborean sorcerers are highly prized in the northern wilds, but they are little respected in Hyborian lands.
Other thing that i wanna suggest is that if Hyperboreans have a natural increased resistance to cold(+40%) and a natural weakness to fire(-40%), it will make then fell more unique. Also, a natural resistance to unholly(+20%)