could be an option except that airlines are all about making profit. Installing special seats will be inflexible (what if no fat people are on a flight), while just charging for two seats will give them a nice little cashflow.
I always thought that it would be more fair to just weigh the people, with their luggage, at the gate and charge the ticket accordingly.
Make flying flexible.
Be slim and travel with nothing - pay almost nothing, except a very basic fare.
Be fat and travel with 50kg of lugagge - pay a lot!!!
It could give people options that never existed before. Maybe you are a backpacker travelling to south america. You take no luggage. Your ticket becomes dirt cheap. You use the extra money to buy a backpack at the destination.
It could give rise to local incentives to the economy. You arrive in a city. You see a store selling new and second-hand backpacks etc.
Or you sell toothbrushes. etc.