I'd like to pitch the same idea I proposed in the past: Prototypes. It fits with any genre and allows the builder to maybe back date a kit from the stash, or build a subject that might be a little off the norm.

Examples might include;

Paper project tanks or Soviet "Object" tanks or NATO counter designs

X-planes, Luft 46, racers or the Space Shuttle Enterprise

Concept cars, Indy Cars, Customs

The Star Wars Expanded Universe has got to awash in one of a kind, experimental ships, speeders and walkers...how about the movie concept sketch books and illustrations that lead to the designs we see in the theater...

Extra guide lines would have to be fleshed out, but it allows everyone to particate in the same GB, and still allows you to build in your genre.

Just a suggestion.