I'm in total agreement with @Louise_G0325 . We need werewolves to round out the common supernaturals, and anything after that is just gravy.
What I wouldn't mind seeing after that is Mermaids, Real Plantsims, and expanded Ghosts.
I'd LOVE to see ghosts get some more attention, such as giving them haunting habits or rooms that they're fixated on haunting, as well as things like items they're obsessed with or attached to that that bind them to certain locations and whatnot. This would be a really fun thing to expand on in the game, especially if it could add séances and the ghost hunter active profession to the game.