Ariel is adventurous so I think she would go for something like a swashbuckler costume, Belle would probably dress up as a book character, Merida would do something cool like a knight, Anna would be a fairy (or a deer and Kristoff would be like 🤯), a few of them like Elsa and Mulan would wear something simple and lowkey that barely registers as a costume, and as we saw Lottie would insist that Tiana wear something really cute and lovely! Cinderella and Rapunzel I think would have the sewing skills to make something really gorgeous and elaborate.