I guess that the best answer would be a RTF weight, i.e. all models must weigh under 12 lbs with fuel or batteries.
However would electrics complain that the glow guys have an advantage by losing weight duing the flight? Would glow guys cry foul because thier planes lose weight duing a flight? Would a higher takeoff weight help the guys that want to try petrol because they don't need as much fuel as the glow planes, allowing a slightly heavier motor? It seems to me that this rule would help to level the field, and keep the current designs competitive and keeping the bipes out.