Honestly, I think this question comes up every year.
The fastball-down-the-middle response is that Halloween calls for the Halloweeners! Psychotic pyromaniacs with a side of fire damage are perfect for the holidays, and a good introduction to the less-than-lawful nature of the Sixth World. A campaign could be based around the mysterious nature of their leader, Nightmare, who rebuilt the Halloweeners after they were slaughtered by the Ancients in 2069. Nightmare wears black leathers, a glowing skull mask, and orange bandanas on the arms. Its aura is masked and looks like it’s on fire on the astral. Male or female or neither? Nobody knows. Rumors about Nightmare's nature: free spirit, vampire, Nephilim (a pro shadowrunner). Maybe the Ancients want Nightmare out of the picture, but what if Nightmare is in bed (not literally) with Urubia?? (Emerald Shadows hints at this.) Ooo! The runners could find themselves trapped between pointy ears on the left and Molotov cocktails on the right.
Ghouls are another tried-and-true staple for Halloween. You can do a ghoul hunt against feral ghouls, as technically there is still a bounty on ghouls in both the UCAS and CAS (¥1,500 each, in both countries). Ghoul rights activities will be horrified, it's true, and may try to interfere or smear anyone trying to redeem said bounties. But what about those ghouls gobbling up (relatively) innocent squatters? Alternatively, you could work in the 162s, a ghoul gang with ties to Tamanous and other organizations that readily lend themselves to a horror-themed campaign.
Shedim are another staple, but I'm going to be honest that I've never used them in all my years of GMing. Not sure why.
