Like said before, there are some hidden 'gems' in the WW2 setting that haven't been done yet: The Finnish-Russian war, the second Chino-Japanese war, French resistance, the failed landing at Dieppe. I'd love to see some more grit and mature game, not the usual FPS, but really in your face and not always victory. Playing as a German or Japanese soldier would also be a neat idea, someone who is human and does not agree with what is going on. Operation Barbarossa is still the biggest militairy operation in history, and yet as far as I know not a single game has used it.
I even already have an idea for this some kind of mission (but way more serious of course), like the AR-71 one in COD Blops. Instead you are a bomber of an B-17 at the raid of Dresden, while on the ground you are a German civilian who has been called (forced) upon for anti-air. He quickly gives up while he sees the horror that is happening. You know, a game that makes you think about war.
But what I'd rather want more then another WW2 game is a WW1. But to make a succesfull game in this setting (like said before, its not capable for the fast paced FPS), it would need to be the darkest and serious FPS to date. You could play as a miner in the battle of Messines, as a soldier who has to hold at Verdun, someone from a connaissance squad at night. But it would need to focus on the human aspect of war, and the horror that WW1 was (I think from a psychological, soldier POV this one was way worse then WW2)