Conflicts that need to be explored more in gaming are the Civil War, maybe even the Spanish American war, World War I (especially), and maybe even the Korean War as well and perhaps black operations of the time period between the period after WWII and when the majority of the events of Black Ops takes place, if we're talking about another CoD sequel. (that would be 1968, considering that only like two missions are set in 1963)
Because Treyarch (because Infinity Ward won't stop making Modern Warfare sequels) can easily CODify any of these war settings, it would make Call of Duty possibly worth revisiting for us that have grown to hate the series and actually give something new to the people that still buy it anually, be it a well-known commentator or typical raging squeaker.