Airlines are very sensitive to the costs of their operations, so reducing the weight of aircraft is important.
Today’s invention is therefore a hammock-like seat for airlines with a much smaller mass than a conventional seat.
This would be made of a netting bag with a fold-down seat unit built-in.
A passenger could carry this aboard themselves and attach it to a hook in the roof.
The attachment might include a spring/damper/hinge unit to limit the amount of motion during eg turbulence. There might also be an elasticated cord between seat and floor.
Such a design would also speed boarding and egress, since the seats are easy to move past, automatically reverting to a narrow rope shape when not in use.