lets see here areas not covered for cod that would work.
Vietnam (no black ops 1 was not entirely vietnam, i mean a whole game for and focusing on the vietnam war and that time period).
KOREA, seriously this was a way more bloody war than vietnam was and it gets 0 attention
World War I
Iran Iraq war, would be interesting seeing how it was a modern style wwI.
Desert Storm
Operation Just Cause
Somalia, good cause 90s plus can re create black hawk down