Having them in a one pack would make sense as both of these occults have magic that is connected to nature and elements.
There are two ways how I imagine elves to work - either on its own like vampires and others, or together with fairies in one pack. Other than fairies I do believe that it is time for The Sims 4 to introoduce own occult we never had before - Elves People have been speculating what other occult we could get after werewolves.
