1. Add LHR
2. Weather affecting aircraft (e.g you have to land/take off planes according to the wind direction)
3. Having to tell pilots info, like weather/visibility/runway conditions, contact frequencies, ect
4. the ability to make the game as close to realism as they want
5. more vehicles on the tarmac, like ops, buses, ect
6. have like a career mode, like having a real timetable from the past year, the ability to get fired, change airports, and seasonal patterns, this will be hard to implement but if done well, itll be very good
7. Savegames
8. Pilots telling you more stuff, e.g when they're at the side of a runway, they tell you or when they've taken off they also tell you (but maybe the option to turn off this extra dialogue)
idk how hard some of these are to implement, im assuming its not that easy, but just some ideas
ill edit this when i think of more stuff