I would like to add these things:

1. Better spacing between approaches, they are sometimes so tight that the only one aircraft per runway rule (lineups behind departing or landing aircraft are excluded from this) is impossible to obey.

2. Less dead space in airport design. The default airports have lots of areas that are completely unused (and some of them also have nonsensical layouts, but that's a different story), and even the realistic airports got this issue due to the airplane models being downsized. Which brings me to

3. Properly sized aircraft models. Not everyone might be aware of this, but the airplane models are all shrunk compared to the environment around them. This can be visible when widebody aircraft are on the runway and the wingtips are still quite far away from the grass, or when you complain the gate lineup in Denpasar to the way it would look IRL. There should be nowhere near this much space between parking positions. This downscaling (that fortunately appears to be consistent among all aircraft types) was probably done to make Gran Canaria (the only airport without ridiculously much space between widebody wingtips) work as a widebody hub (they don't really get a lot of that IRL). Understandable, but kinda frustrating once you notice it.

4. Wind direction changes. This would require quite some effort to implement, but I'd like to have runway changes every couple hours or so.

5. Proper flight schedules and the option to edit those. If you ever take a look a look at the flight plan in the map menu, you will notice that different airlines serve different destinations. This is neat, but unfortunately the destination selection is.... rather random and not well researched. Etihad for example has their only hub it Abu Dhabi, so it would make sense for all their flights to be heading there, right? Nope, they also fly to Dubai and Sharjah (and it's vice versa for Emirates). Iran Air serves Chinese destinations for some reason. Indonesian airlines will be heading to Bali..... while already playing at this very airport. The list is long and I'd love the option to create my own flight plans to fix this.

I could think of many more things, but I'll leave it at this for now

It has been a long never seen update