Let's assume that the National/Regional thing goes away and every GCR-recognized class is eligible for the Runoffs - here is what I would do:

'Reward' the top 10-15 classes in average participation with their own Runoffs run-group. The bottom 'half' still get a chance to run for the gold, but they must do so by sharing a track with someone else. Split starts a must...but until numbers warrant it, you pay the price of getting invited by having to share the track with another class or classs.

This way you can fit the Runoffs into a more compact time frame, which is a common complaint.