Gamera,
All it takes is one guy with an idea. This isn't something that's done quickly. I know when I've done this, the idea has been in my mind for awhile. Once I've come up with a theme, let's say Mustangs of the World, I'll decide on the scale (1/72) and the model(s) to be used (for the D/K Tamiya, for the B/C Academy). Then I'll find out how many countries flew the plane and what decals are available for the various countries. Once that's done, I'll make a list for people to sign up on. I'll then present it to the club for their consideration. If it's a go, then it's like herding cats. You'll also have to decide on the base because it is the only category in which the base gets judged. We have a rule in our club, if you're in charge, it's your responsibility to get it to the Nationals. While it's a pain in the tuckus, it's also alot of fun.