Yes, an ATC-System, a fuel and weight system, ground services, metar and multiplayer are the most popular and most important requests.
Nevertheless, I feel like, there’s something that is talked about too little and that is special effects. A game feels alive when there are these effects.
Here, I’ll list some that are quite important:
1. Smoke after touch down. This is the sign that your plane has made contact with the ground. It would make watching the landings in replay mode much more enjoyable.
2. The heat behind the engines that makes the air look blurry.
3. Vortices. The clouds that build up over the wings. Beautiful to witness in person.
4. Breaking the speed of sound. It would make flying fighter jets way more exciting (even tho you wouldn’t even notice it as a pilot).
There’s probably much more that you can come up with:)