Actually, may not be a bad idea. US could revolutionize the way air travel is priced by charging a unit price per pound. So you, your carryons, and your checked bags are all loaded up on a scale. Then you pay by the pound. So for example, BOS-LAX could be priced at $2 per pound for a W fare, but $8 per pound for a Y fare.

This would be a more accurate reflection of costs, and would make US attractive to lighter, more profitable people. The FFP could offer free gym access as a perk. And they could add treadmills to the lounges.