What's the next spec class? Good question.
Mazda and Honda are working with the SCCA to get the "B-Spec" class going. These are the small, economical B-spec cars such as the Mazda2, Honda Fit, Ford Fiesta, Nissan Versa, Chevy Cruze, Fiat 500, etc.
In 2012, they'll run in SSC and should have their own SCCA class (B-Spec) by 2013.
Time will tell if the class takes off. As the cars are newish, they're not cheap by club racing standards.
My daily driver is a 2011 Mazda2. It's a blast to drive, even if it has maybe 95 hp at the front wheels (on a good day).