I think the less restrictive the theme, the more participation will happen, and the larger the appeal to onlookers. I dont have any concrete ideas outside of having a single point of inspiration, something like a photo, or painting maybe? The builders would only have to state how they were inspired, but could build anything they wanted based on the inspiration (the lowest pitch stipulation should probably be included...).
Or maybe a function theme like "buckers, no hum allowed!" Builders could make any kind of bass, provided the electronics didn't hum. Traditional dual coils, hidden dummy coils, piezo, midi, whatever, as long as it dont hum.
(If I've used up my pass for naming a build off just let me know and I'll see myself out

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