Just a few thoughts:
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[*]Halloween costumes become real
[*]Zombies, vampires, and monsters appear and create an apocalypse
[*]Group stays in a haunted house over Halloween.  Monster ensues
[*] The goblins and bogans rise as Samhain, day of the unseelie, begins.  Heros have to dress as monsters to fit in and get to Lord Samhain.  Could be modern our medical fantasy
[/List]