How about you start the game like CoD, just fighting soldiers...
but then some of your own allies start going crazy slowly, losing their grip on reality,
killing themselves in strange ways...
then we go through some of the Cthulu Mythos by fighting out way up the ranks.
The point wouldn't be to beat the enemy as much as witness they're raw undefeateble power.
You would be able to kill the weaker of them, with difficulty of course...
but then it would turn more into a game of desperation, trying fervently not to be seen by the monsters, running and hiding, slowly realizing that no matter where you go, they will find you.
Then the end of the game is you seeing Cthulu, trying to kill him emptying your clip, knowing it will doing nothing, tears streaming down your face as you go insane, then a lower class monster hits you and goes after another person, and you lay on a distant hill (great view) almost dying.
Then while slowly bleeding out, you see the armies of the world dying at Cthulus hand as jets kamikazi themselves uselessly at him and missiles are erased from existence with a thought, (cutscene here btw after you got hurt), until he simply destroys them all, in x way. (ending is hard to think up, any ideas)
Hows that for a game? Thoughts?