Wait, 100kg is 15.7473044 stone.
Almost everyone I know that is >5'8" and goes to the gym is almost that weight, let alone baggage.
Actually yeah, 100kg may be slightly low, say 120kg?
Goes to the gym? You mean bodybuilds. Although some people may fork out more to go but I disagree that I should be confined to 15-20kg baggage when i weigh 2-5st less than someone else.
(I'm actually semi serious about this, it makes sense to charge heavier people, who will use more fuel.)
If your overall weight is more then you pay an excess weight charge, easyjet and ryanair already allow you to add excess weight onto your baggage so it wouldn't be too difficult to impliment.