There'll be all sorts of death stuff coming out of Shadespire, surely. Boney minions, heroes, bosses. They will of course make it a gateway drug to AOS by making the new models available for use in both games.
Personally I'll leave it for now as GW has supplied plenty of other stuff for now to keep me busy. (Firestorm appeals more as its refreshingly free of Stormcast, and gives goodies for goblins.)
These 'Death Cults' must be mortal humans wishing to become immortal by letting the wraiths have their way with them. For the wraiths I guess its just recruitment policy to raid the willing mortals. Why don't the cultists just leave the city and setup in caves etc, like in Diablo? Maybe they do, and help the wraiths with intelligence, e.g. ways into the mortal cities...