I don't know why so many people are saying that there wouldn't be gameplay for werewolves. It seems pretty obvious to me.
Point 1: Look at the other occults we have. With the exception of Vampire and Spellcasters, most occults are just a transformation and a tiny handful of powers. It wouldn't be at all out of place to have werewolves also be a transformation (Wolfman style) with a few powers and interactions, just like Mermaids and Aliens. This route makes the most sense if Werewolves get added as part of a big pack that isn't necessarily about them, like how City Living and Island Living were. Customizations would be similar to any of the werewolf or catperson mods out there, with fur patterns, noses, whiskers, ears, claws, fangs, etc.
Point 2: There are tons of things werewolves could do! Interactions and moods are especially easy: your Sim can howl at the moon, sniff other Sims for info, have unique pet interactions, "hunt" Sims using a unique fight animation, get good or bad moods from needing to hunt or successful hunts, eating raw meat, peeing outside on trees and fire hydrants, etc. The game can add wolfsbane and use it in similar ways as garlic in Vampires. There could be special Sentiments you get for having a pack and with members of your Pack, or bad Sentiments from rival Werewolves if they want to go the whole "Alpha Wolf" route. They can give werewolves a new needs bar just like Vampires that tracks their need to transform, which can be slaked by going out and hunting as a werewolf. Random transformations at night and vulnerability to wolfsbane could be used as motivation for buying into a Werewolf skill tree. Hunting itself could be used to fill the Hunger need. There are lots of possibilities.
I don't know much about fairies, since I never had a Sims game with them before, so I can't speculate on how Sims fairies would work. But when you look at something like Aliens, who only have maybe 3 interactions and a special appearance, it seems silly to say that any occult, whether it be werewolves or fairies, couldn't exist in the game or be fun to play. The occults we already have aren't that in depth, but adding them gives players more stories to tell and ways to play their Sims. I'd be happy to get either.
(But mostly werewolves. I love werewolves.)