Not to stir the pot or anything, but I was thinking about Sims fairies today and how they might be done. There's no way that a fairy pack will satisfy every occult Simmer (TS4 has yet to give us any super dark occults and I doubt they're going to start now, for example). So I tried to brainstorm things that make sense within the realm of what TS4 has given us with other occults.
Simple Fairies: Like aliens or mermaids are in the game right now. These fairies would likely be included as a side feature in a larger pack not really focused on them. (Honestly, for fairies, I think this is a likely scenario, since they're an easy thing to package into a broad variety of packs like woodland, medieval, or fantasy.) Fairies include different unique customizations, like wings and eyes and makeup. They use a unique energy bar to cast spells, and have both Charisma interactions (Tinkerbell style spells and good magic) and Mischief interactions (pranks and more malicious spells). They have unique moods such as gaining energy under moonlight like in earlier games, and unique whims like wanting to use their fairy powers. They will likely have be good at gardening, too, like in earlier games. Normal Sims can become Fairies by doing a moonlight ritual at a fairy ring, stone circle, or strange tree somewhere in the pack's included world.
Complex Fairies: If we get a full pack like Vampires or Realm of Magic, we could have all of the above and a few more features. In the perk tree, you could decide what path your fairy can go. They could be an Unseelie style dark fairy, focusing on malicious spells and pranks. They can be a Seelie/Tinkerbell style good fairy, doing blessing spells and helping people. They have a path for previous game's fairy features like living longer or replenishing spell power with moonlight. They also have a path for their green thumb/nature powers, since that was something they could do in previous games. I think it would be a cool feature of this path in particular if Fairies could turn Sims into PlantSims, since people have also been asking for a PlantSim refresh, and these two things would fit together nicely. I think another nice addition not found in previous Sims games would be if fairies also had a boost to music skills and dancing as well as gardening, since these are a big part of fairy folklore.
Complex Fairies From The Other Side: I think a wonderful addition to a fairy pack would be a trait for The Sight. Sims with this trait could identify fairies around them, like the special interactions to identify mermaids in Island Living, but with a pack. Sims who look too hard for the Fair Folk could occasionally be abducted by them (missing for an hour or so in game) and come back with unique moods from their experience, similar to Scientists and Aliens in Get To Work. Sims who wander into the aforementioned fairy ring/stone circle/strange grove would have an extra chance to have an encounter with the Fair Folk.
I think these ideas are all fairly well in reason to expect for a TS4-style pack. I tried to base all of these on features we've already seen in the game or similar. I think hoping for anything more complex than this will inevitably lead people to be disappointed.