Another extreme option is to post in the classifieds the six themes as separate leagues. People can then sign up for just one of the leagues. You could easily get 3 more people to sign up for each of the 6 leagues (i.e., 18 people, although some folks may sign up for 2 or even 3 leagues). They wouldn't be part of the Championship and the overall prize money would have to be modified accordingly.
That being said, I bet we get this filled soon.