I though as @Jettuh posted his little thing here I would too…
I would like to have a small arrow at the placeable stuff like counters so it will be more clear, in what direction an item looks. Also, you could color-code certain squares to indicate, where a passenger or employee will be standing at the item. This way you could save some trouble with placement only to get XX cannot get to work messages.
I would like to see more liveries for the GA Aircraft. Shouldn´t be too much work but would make it more enjoyable to play at a GA Airport.
A follow mode for the camera. So the view keeps following a certain aircraft/passenger.
Barriers/cordons to cordon off areas (sort of indoor fences). To control passenger flow, like used at real airports.