Costume category in CAS for sure (tho I feel like that should be a base game update)
More classic costumes like Dracula, Frankenstein, Ghost for both child and adult sims
Being able to go trick or treating
cross pack play, besides the pumpkins, maybe be able to knit your own/your childs costume with NK, adding new horror movies to movie night (and the related posters)
a spooky day festival sounds cool too, could also include a costume contest
add horror books that make people scared after reading them (also being able to write horror)
add a horror tv channel
add horror music station (like more instrumental, classic horror themed than the spooky station)
Ouija board gameplay object
add graveyard
telling spooky stories (sitting in the dark in a circle with flashlight lighting up the face)
spooky aspiration (could include, besides scaring sims, to read horror books/watch horror movies/channel, throw a perfect spooky party, use Ouija board successfully, spend a night at the graveyard, win costume contest...reward could be "enjoys scary things" so when the sim gets scared it makes them happy/adrenaline rush and they wont have the urge to run away and hide but are rather curious about scary things)