Rory: Probably something that project the illusion of confidence and security, so... police officer? Armored knight? Atlas airship? Not quite sure on this one.
Sean: Zombie Willy Wonka. Puts everyone else off candy for the evening, thus more for him!
Alec: Bread Geist... because why not?
Cordell: Herself! No, seriously, Halloween is probably the one day a year she could walk into Vale and mingle without drawing attention. Maybe add an eyepatch, just for funsies.
Prowler: Literally does not need a costume -- just walk down the street in shorts with his full-body 3rd degree burns showing, trailing plumes of smoke with each footstep. On a tamer note, would make for one badass mummy.