IMO, asking for offline AI is too much. But offline play should be an option to train more things than just takeoffs and landings. IMO, "offline" is just the perfect option to train our aiming skills whithout the need of being connected for hours to TA or H/H.
The simplest way I see to implement this is as follows:
1 - Put a lot drones along the map, tanks, fighters and bombers. This seems simple, just a X,Y,Z coordinate per drone and rendering it.
2 - Give each kind of drone a pre recorded path. This should be simple also. Just take off with a plane, turn record ON, do several maneouvers (climbs, dives, turns), turn record OFF. On the recorded file we have all the info to translate the moves to a drone.
3 - Animate those drones. Just like when we play a recorded film on AH video.
4 - Set up an option to "rearm/refuel" your plane in-flight during offline, for example ALT-W.
5 - Give an option to start at a desired heigh over the selected field in offline missions.