Separating out Supernaturals from SIMS 3 into Realms of Magic and Vampires in SIMS 4 seemed to be a downgrade. If they want to continue with the separation, then Realms of Magic should include Genies and Fairies with an upgraded power set and systems. Vampires could include Werewolves and Mummies. I think that Ghosts and PlantSims should be part of the base pack, but if they wanted to make a 3rd pack for Supernaturals, I could see them making one with Ghosts, PlantSims, and Mermaids.
I think that there should be a Superheroes Pack. If they wanted to add Robots and Aliens to it, that would be a good way to add content with things that they have already released in previous seasons.