I feel like witches would be their own game pack. Werewolves would probably go with Pets (which is hinted to arrive soon), while fairies and mermaids could make up an expansion (assuming that the same level of detail was given to each as the vampires.)
This is just my speculation though. Those seem to be the main four, if you take the statue hints into account, and there have been hints in the game before of things to come. Wasn't there a display case version of the freeze ray before they actually did Get to Work?
Anyway, only time will tell and I can only hope. I'm really curious how they would do mermaids; you can't swim in the ocean in the Sims 4, so they would need their own world.