Moon cycles. Introduce those to influence werewolves' ability to stay in their human form or be compelled to transform into their second form. As they "level up", they get better at control and could have a perk or skill on the skill tree that allows them to transform at will.
Have the option of the transformation being either a full wolf or more hybrid shape.
Bring a pack hierarchy within the system (a variation of clubs maybe, with more assigned roles than Club Leader) and different abilities. Yes, I'm aware that the wolf pack hierarchy theory has been debunked but in fiction it's still fun to adhere to.
Maybe make it a shapeshifter option so it's not only werwolves but various other possible animals that a Sim could transform into. Have them be able to communicate with dogs when they're in their wolf form.
As for transforming, I'd love if the devs delved into all the legends about how one can become a shifter: the obvious bite, but also drinking water from a paw or footprint left behind by another wolf (and I'm sure there are more).
I don't know if it's viable to have a full skill tree UI for werewolves but it might be possible with a variety of shapeshifter types. One branch leading to a full animal shift, one for a hybrid type, one for a full-on Underworld-style werewolf. Perks for enhanced senses would be nice too (sensing emotions in other Sims), "curses" like uncontrollable shifting or stronger sensitivity to the moon.
I'd prefer them getting their own GP, but a lot of the above could still be possible if they were a part of an EP.
