This idea of charging by weight has discrimination, ethics and lawsuits written all over it. I think, like chuboy, there should be a base fare (like now) where you get a percentage discount for every lets say X amount of weight you are under the standard (the standard could be set universally).
If you're a business traveler going for a day round trip for example and don't have any luggage, you can use that weight savings to your body weight to save. If you re a child and weigh very little, you pay less. If you travel frequently, you get forgiven for your weight more you fly etc.
It only makes sense to do this and it will happen because you can only limit carry on and check in luggage so far. Weight has a direct impact on fuel savings, and i don't see electric and solar powered 300 seat passenger planes in this lifetime. If you pay more or less should be determined when you weigh your baggage, everyone step onto belt as well
