It seams like every week the airlines a adding new fees on top of the normal ticket price. At first they were small charges, like charging to buy a ticket, or change a ticket. Then they started charging for heavy baggage. Now the trend is to charge for checked baggage. There is talk in the industry that the airlines might start charging for Carry-on bags.
But what I think is coming is a day where the airlines start charge for passengers tickets by weight. After all a big heavy person takes up more room in the airplanes cabin. Airline fuel consumption is determined by weight. The heaver a plane is the more fuel it will burn. Air freight prices are determined by size and weight of the package. The larger a package is the more it cost to ship it by air. Same goes for weight, the packages can be small, but if it is heavy it cost more to ship.
So do not be supprised that some day, in the future, you are asked to stand on a scale at the airline ticket counter so the ticket agent can determin the cost of your ticket.