On one hand, I like the enclosure idea. Everyone buy the same PE enclosure (.25, .38, .75, whatever) and then load it with drivers. Maybe put the limit at $250. The competition would then be on choosing drivers, crossover points, components.
On the other hand, choosing a single driver means that the competition would be about building an enclosure. However, we could still allow people to augment the designs with other drivers. For example, even though the Jordan speakers I have are "full range", they sound better with the Aurum Cantus ribbon tweeters. This way, we could still have a range of different approaches to using the same driver.