I would limit the planes to a weight limit vs. a wing span. If a beginner has a tiger 60, Kougar, or even a DB ARF and sees he has to compete against a full on 80" Extra 300, he will be less than thrilled to compete against a "competition plane". Use the same 11 pound limit that pattern currently uses, or maybe even 12, and leave it at that. That fits many many sport planes that are currently flying at any given field. For the most part, the intimidation factor needs to be lowered to get average sunday flyers involved.
One reason saturday night drag racing works so well is that they have they a street class that guys run the family car, truck, etc without having to run up against a 10 second camaro in their stock VW golf. Guys that already have the fast cars will not run that class due to egos, and that would work the same way here. If somebody already a "real pattern plane" they will fly sportsman since they have the plane.