I'd love to see a Faerie pack. Based on the folklore, where Fae were actually dangerous.
In the light of the new magic realm, there could be a Fae realm. You'd need to be careful not to eat the food or dance if you were a human.
The Fae Courts: Unseelie, Seelie and Solitary. Different types of Fae have different abilities and natures. You could go to them for favours, for a price. Which would rarely ever work out your way, but you could get lucky. And isn't that worth it?
Ability to trap imps and keep them as a sort of familiar.
Pseudonym names for Fae, as giving someone their real name allows that person to control them. There could be a way to find their real name underhandedly.
Also being able to bargain your own name (a terrible idea, don't do it) to say, bring back the dead, or become famous, with more chance of it going wrong.
Some humans would have the ability to see Fae and imps in the real world. And be more susceptible to Fae related 'incidents'.
Other general ideas I'll throw out:
Changelings, Glamours, Time moving slower in the Fae Realm, Faerie circles and toadstools, Fae powers, Silver good, Iron bad, make jewellery of either to protect yourself from or attract Fae.
I want to genuinely fear that if they enter the realm, my human sims may be unable to come back. And, I don't know how to politely put this but, enchanted human servants anyone?