People should buy an allowance with their seat, say 120kg. If you weigh 80kg you can take 40kg luggage. If you weigh 120kg, no luggage or pay excess, and if you weigh 130kg you pay excess baggage on your own weight. This would make it fairer for all and encourage fat pax to pay for business class, where they might get 200kg allowance, so no excess to pay for their own weight.
Passport could contain info on weight, or ID card when we get them.