Some kind of covering that we can place on the existing roof to give it a bit more decoration. It would probably work like the flooring tiles do currently, just on the roof. There could even be special designs such as the Tom Bradley Terminal or one like Madrid.
(Honestly, these ventilation shafts are really ugly)
Why it should be implemented:
Currently, the architecture is really bad (I’ve never even seen an airport without some kind of roof) and these roof tiles could add a lot more creativity to the game.