Here is some input from my side.
First of all great game, I have been enjoying this game for more than 30 hours since release.
One annoying feat, that others have also pointed out, is when flights have to be planned. I heard that this system was implemented in a late stage during development. However, it could be tweaked to hold slots. Slots are used by airports to sell landing and takeoff time to airlines. Likewise in the planner that is used now, a slot is automatically assigned to an airline, aircraft type and passenger amounts etc. Why not determine only “spaces” (slots) and let airlines fill them their-selves? (Or by an operations manager of staff (?)) Still gives full control about the times and gates to the CEO, minus the hassle when he only has to define slots that are repeated every day/week one time only. (Or when adding stands)
The capacity of each stand can be maximised when airlines are allowed to stay overnight. The takeoff and landing restriction will still hold place, but the aircraft just stays on the stand. Just like what happens with GA aircraft. This allows airlines (for example) to land at 21:00 and depart 05:40-06:00 the following day. The departure runway will become wayy more effective in the morning.
In line with the last item, why not have homebased airlines? A contract could be signed like now happens, but only with a homebased airline also to occupy gates at night. Maybe add an airport hangar for the aircraft to spawn out of or have maintenance or something.
I hope these ideas are useful and can be built upon. If someone comes up with an addition or a twist to these ideas, be welcome to post below!