Halloween a time where the ghouls of the night to dance in the moondlight...
Well the people really suited for halloween are Necromancers, Dark Avengers, Spellhowlers, Phantom SUmmoners, Kamael as they are the only dark classes.
How about add to the spirit. Halloween decorations in town, and for like a week or so, special transformation scrolls drop from mobs allow you to transform into ghosts, zombies, skeletons, demons, they have these types of characters in game as well all know. And maybe like mini buff candies.