I'm playing at Heathrow which is heavily slot constrained.
When creating a new route it would be amassive help if the hours and minutes in the "time of departure" selection box were coloured to indicate whether a slot was available at that time. I'd suggest the hour be green if there are any slots available in that hour, and red otherwise. Ditto with the minutes, if there is a slot at that hour and minutes, green, otherwise minute should be red.
This would help a lot when moving a flight up and down to try and squash it into an airport with next to no slots.
08 10
08 20
08 30
If some people don't like the design it could revert to the normal white and black if selected in the game options page.
Simon