Vampire, she already the fangs.
Cute witch is always nice too.
Cheerleader is a 10/10 lewd costume but it's not really Halloween material if you're still into the spooky part of it, which Star ought to be.>Marco
His dancer outfit is just too good.
Knight/Conquistador seems like it would be too clunky.
It's hard to picture what a Djinn/Genie outfit would be, but I picture him (nearly) shirtless and that's nice but kind of weird since Marco's supposed to be pretty insecure about that sort of thing.