- If one of the themes in your claimed category is canon for your fandom -- e.g., you've claimed Captain Jack Sparrow in the Cosplay category, which contains "pirates" -- then you may either put a creative spin on the theme itself (interstellar pirates! Vikings! highwaymen!) to make it AU, or else substitute in a "writer's/artist's choice" for that one theme.
Are you also allowed to substitute a writer's/artist's choice if you just don't like one of the themes in your claimed category?