Had the same idea today for a racing/flight sim!

I have no idea what games output for motion, that would help to know, but whatever it is (and i am sure no 2 games use the same output, but maybe through directX...), we would need just translating those to waypoints and from those to gcode.
Taping into games collision detection is out of the question by now as the coding on new games is absolutely terrible and utterly useless.
But, games that have display/gauges for pitch/tilt/angle of attack, should be much easier if they also output somehow somewhere those values.
Then there is the interpreter/LinuxCNC reading those values and sending motion, but should be doable, there are some "camera to motion" things working already somewhere on this forum.
Maybe use that and point the camera at the screen...