Theme: Once Upon a Mask
This is NOT intended to be a Cosplay theme.
If you've not seen the Masked Singer, any country, the participants are in elaborate costumes to hide their identities, and then they sing on stage. The costumes range from various Monsters and all-white vaguely demonic-looking Unicorn, to steampunk Fox and various fruit or food-themed creatures. It's aired in 11 different countries, and since each season has a good number of contestants, there are a plethora of costumes to take inspiration from. They do not generally show the contestant's actual skin or hair at all, to minimize clues.
Link the The Masked Singer franchise Wiki page.
TL;DR - Pick a "mask", dress up however, be creative, and then add stage components.