Here is my suggestion. Sub-forums for regionals that have 500 pilots or more. That would be 13 of the 25 listed. (As a comparision, Majors have 8, which may one day become 10 if Frontier and Allegiant get added).

Here would be my proposed list:
Air Wisconsin
Compass
Endeavor
Envoy
ExpressJet
GoJet
Horizon
Mesa
Piedmont
PSA
Republic
SkyWest
Trans State

This would cover 95% of the pilots. My guess 80% of the threads (there are always some general topics plus the smaller regionals).

Your thoughts?