IMO one of the things closer to the realm of possibility than an FPS, yet just as hard to make, is a *visually correct* flight simulator.
Requirements:
-Throttle control. Remember, airplanes have a speed requirement to stay in air, so don't make this a really low number.
-Collision (Flight sim collision is generally less costly)
-First Person Cockpit view, First Person, and Third Person view.
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Flight instruments. All of the meters except for the course deviation meter and the radio magnetic indicator are necessary.
-Do a barrel roll!
[Optional]
-Interesting flight instrument layout. I didn't say the flight instruments had to be physical and attached to the plane. Just that they had to be present and visible.
-Targeting System. This can be with or without lock-on targeting. Remember, as a general rule of thumb missiles have guidance systems and rockets don't. Gravity bombs, even if they have guidance, are still only considered bombs.
-Environment. What matters is the flight, not whether or not you're flying over an oil spill or if you're flying right next to a giant radio tower that is jamming your flight controls.
-Landing, various other miscellaneous methods such as changing the position of landing gear and rolling out the evacuation slide.
-Other planes + AI. I recommend not making AI. You will invariably flop it, and Alice isn't boss enough to support half decent aerial dogfights even if you didn't.
Example:
I can see an altimeter, a turn indicator + crosshair (targeting system), and a compass (radar).