I keep waiting for fairies for ages now, and there was a survey and hints and what not, but somehow this seems to be a forgotten occult.
I can see a huge potential for this occult and in my opinion why re-invent the wheel.
Lets just take a reputation for instance - fairies could be good or bad based on their decisions and actions and have a unique interactions with other occult or non occult sims. Of course I would expect a lore to be build around it, their own world, abilities much like for werewolves and special look for both masculine and feminine looking sims.
Another possibility easily compatible with the (good/bad) reputation or simply choosing to follow dark/ light path would be incorporating seasons, meaning being able to manipulate weather or lets say a small example if the fairy gets angry or is bad/on dark path can strike other sim with lightning or make it rain only on that specific sim and opposite if good/on light path the fairy can make the plant blossom or bare fruit immediately or fix a bad relationship.
I would also love to see some interactions with the animals that are already there like become friends with killer rabbit as a dark fairy and be able to adopt that rabbit as companion or something like that. Light fairy could have "better than best friend" ability with animals, so that for example cow would always give some special milk when being milked by a fairy. Or the light fairy could help sims conceive twins or triplets and the dark fairy could cause infertility for example.
Even with other occults - if at the highest level they could either curse someone with becoming werewolf or wampire or free them from this curse and that's just one out of many possible occult interactions.
There is a lot that could be done with fairies it just requires a bit of imagination and pixie dust 😉