I think costumes to dress up like monsters in the FF bestiary would be great. There could be Cactaur costumes, Bomb costumes, Tonberry costumes (the slasher XD), Marlboro costumes, Goblin costumes etc. Maybe even let us dress up like Primals!

If we wanted to break the 4th wall a bit, 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. Imagine how awesome a Kefka costume would be. XD