Flight Planner tabs indicate full schedule
On the days of the week tab in the flight planner there would be a little icon indicating if there are available ATC slots on a given day.
It’d be useful to see if there is an available gap at a stand as well, but that might be trickier to not clutter the UI with across different plane sizes/international/etc.
Why it should be implemented:
When doing manual assignments, sometimes you fill up to the ATC limit for multiple future days with flights. I find this happens pretty easily in the early game with the 25 flight limit. Therefore, when scheduling newly offered flights, you have to click through multiple days until you find one which has open slots.
This small change would simply save a little effort and provide more information at a glance.