I'm thinking that we are going to see either another Vietnam era game (like black ops) or possibly a future war game.
They won't go modern because MW has that time period on lock-down, WWII has been done to death, and WWI just honestly seems like a WWII game but with less guns.
Maybe the Korean war?
Platinums: Red Dead Redemption, Killzone 2, LittleBigPlanet, Terminator Salvation, Uncharted 1, inFamous Second Son, Rocket League