I like the concept of having to race your Regional to attend the Nationals. That's what they just did over in EFRA for the European Champs. Drivers had to race in their own country's (much like our Regions) championship to be eligible.
Once the turnout at the regionals increases, you can make it a requirement to finish in the top 10 or 20 in order to attend the Nationals. Then the Nats are limited to 150 racers, for example (just like EFRA). A lot more prestigious that way.