Basic: Users can create their own airlines in game, they can fly in single player or multiplayer.
While flying for your airline you cannot use fast forward etc.
Have to use semi realistic landings etc.
Can use CPU pilots for offline etc.
Can do online as well
More advance:
Make it so the planes take on age.
For example if you decide to fly into a storm, or you hit hard on the runway, the plane takes damage you can decide to fix now and save big money later, or risk it.
Even more advance:Add corrosion, and stuff.
If you have bush pilots in your airlines, they will be landing on a lot of dirt-like runways meaning they are more vulnerable to corrosion.
If they can do that, that would be neat rather than using third parties for airlines.