Flight sims, in comparison to the FPS genre, are dead. The games are unpopular and tend to just redo or slightly modify the work that has come before. But there is definately one aspect of flight sims that I'll never forget: the dynamic campaign mission generators! Falcon 4 and F-22 Total Air War, for
example, have these. But no FPS does.
What's the generator like? Well, you basically have the war over a large space of several nations. From there, the AI simulates a huge war. Tanks rumble around and blow up each other, jets scramble off of air bases and fight, bomb, protect. The success and failures of missions determines the future events of the war. Blow up an Army fuel tank, and you might see less tanks around. Crater an airfield runway and planes might not take off of there anymore. Quit the game, come back later, and the world is just as you left it.
FPS games, I think, suffer from an immense community/user requirement for additional replay. While many user MODs and maps offer much more in-depth replay than an AI generated campaign mission would, normally those MODs and maps are pretty rare and of variable quality. And often these maps and MODs are focused entirely on the multiplayer experience, requiring one to have a fast Net connection (especially due to the realistic nature of current games nowadays), a lot of people, etc.
FPS games should try to adopt the dynamic campaign system for additional replay value. (Plus it would be nice to see an FPS game have a decent "wingman" [squadmate] support system...besides the freakishly realistic Rainbow Six games)
Like in Total Air War, the player would start off watching icons moving all over a screen blowing up each other. Tanks are on the roll in sector 5. Pick an anti-tank mission and plot your waypoint towards Sector 5. Equip your troops with weps. Roll out. Kill the tanks, good. Fail, tanks blow up your ammo depot and you got crappier guns the next mission.
Dynamic mission gen would give you new targets, new objectives, and so forth everytime you play the game. The terrain and surroundings may get boring, but if done right, the gameplay will remain fresh and fun :-)