Several cons I know of have yearly themes. Starfest had themes like 1930's gangsters, tropical island and others. I do remember they thought of Mardi Gras one year, but realized handing out beads might present a problem

CONvergence would have ones like classic sci-fi (Buck Rogers, Flash Gordon), bad guys (Darth Vader, Agent Smith), time travel (Dr. Who, the Time Machine) and more.
In both cases, the theme was reflected in that year's artwork, decorations, and staff outfits.
NDK could have some fun, considering various anime:
Mecha, Horror, classic, 80's, samurai, etc.