My #1 costume would be a CUTE MOOGLE!!!!
Love this! I agree with Agoven! We should be able to dress up as something FF related like Moogles, Moombas, Tonberries, Chocobo's etc.

I think it'd be pretty funny if there were costumes to dress up like select characters from previous FF games. They could be said to be heroes and villains from popular childrens stories in Hydaelyn.
I like Zuellni's idea too for dressing up as past FF "stars" lol. We should have monsters like the skeleton swordbearer or skeleton soldiers, rise up from the dead and walk around the cities at different time with bats flying from over head..

And candy to eat, LOT'S of candy