I think the best Kirby ever
would have a 7A motor comparable to the G3/Avalir placed into something resembling the 5xx series. Four row brushroll, HEPA bag, keep the Amodel fan for heavy duty use, 50' 3-wire cord interchangeable with a 30' for residential, heavy-duty wheels, heavy-duty outer bag made of similar quality as the Heritage era bags (thicker, stronger material). Cast aluminum mini-Emptor.
This would be the ultimate commercial machine.
A suction-relief valve on the top of the floor nozzle would be a must. Notches in the floor plate on each side for edge kleening.