Call of Duty originated as a series of World War 2 games, but games set during The Second World War are hardly new. Just going on shooters, Battlefield, Sniper Elite, Brothers in Arms, Medal of Honor, and Day of Defeat all did it, and they're just the ones that try to be somewhat realistic. Some shooters like the Wolfenstein series and Dino D-Day like to add wackier elements to their depictions of World War 2, and some of these games actually pre-date the original Call of Duty.
The developers at Infinity Ward quickly realized the potential for moving away from this setting after they finished Call of Duty 2. That decision resulted in 2008's Call of Duty 4: Modern Warfare. The game was a smash hit and showed that the series did not have to be limited to a single era. Black Ops tried something new by focusing on Cold War espionage in the 1960s.
After that, Black Ops 2 started a period of Call of Duty going in a more science fiction-oriented direction, which continued through Black Ops 3 and 4, Ghosts, Advanced Warfare, and Infinite Warfare, before revisiting World War 2 and going back to the theme of Modern Warfare. But there are still plenty of good time periods that have not been touched on in the games.
5 American Revolution (1765-1791)
At first, this might sound like a strange choice due to the obvious technological limitations. The American Revolution did not exactly have access to automatic weapons, rocket launchers, helicopters, or any of the fancy equipment players get to mess with in other Call of Duty games, but that could actually be used as an advantage.
Replacing assault rifles with flintlock weaponry would change the way players experience Call of Duty. For instance, flintlock weapons can only fire one shot before needing a lengthy reloading process. This would require a bit more strategy in terms of finding just the right moment to fire and the right target, as well as being able to find cover to reload. They could even make a mechanic out of the steps required to reload a musket. This would also encourage new and different strategies in multiplayer.
Some other options for a revolution-themed game could include working artillery, which would also require players to be able to aim and fire at the right target at the right moment, and naval combat, something not usually done in Call of Duty.
4 American Civil War (1861-1865)
There have been a few attempts at Civil War games with mixed results. The History Channel attempted to sponsor several shooters but most of them were poorly received. However, with the production values and attention to detail of the more recent Call of Duty games, it might be possible. The series has attempted to experiment with different time periods but has only gone as far back as World War 2. There are plenty of opportunities to explore older conflicts, especially with the series' fondness for "boots on the ground"-style fighting that would be perfect for an earlier setting.
The Civil War would be a good one for experimenting with an older setting that would change a lot of gameplay. Players would have to work with the limitations of older weapons without being able to rely on air support, drones, or fancy gadgets. But there could still be opportunities to try out different things, like handling artillery and early machine guns, or playing around with cavalry mechanics. If they're feeling really ambitious, they could even incorporate some historical commanders and battles from the war. And of course, the Union and Confederacy would make excellent factions for multiplayer.
3 World War 1 (1914-1918)
World War 1 was one of the most pointless conflicts in history. Everyone was stuck in trenches fighting and killing each other over inches of land, sometimes needing to get very creative when resources ran out. It did not help much that the war saw some major technological innovations that quickly overwhelmed soldiers and generals rooted in Victorian Battle tactics. The establishment of automatic weapons, tanks, submarines, airplanes, and chemical warfare made the whole thing a bigger mess. To top it all off, the whole thing ended with the Treaty of Versailles, which directly set the stage for World War 2.
But even so, there might still be some potential for a game dealing with the war. The trick would be finding a balance between interesting gameplay and representing the brutality of a futile war between two equally bad sides. There would certainly be a variety of weapons that could be incorporated into gameplay. Players could get access to WW1 rifles, pistols, and machine guns. There could even be a level focused on aerial combat in biplanes. For an added dash of realism, the game could also incorporate some real environmental dangers of the war, like trying not to drown in thick mud or accidentally setting off mines.
From a business perspective, this might actually be doable. The success of Battlefield I would seem to indicate that there is indeed a market for a World War 1 game.
2 Korean War (1950-1953)
Black Ops focused mainly on the Cold War as a whole. Even if it did touch on some real-life events like the Bay of Pigs and Vietnam, the game's main interest was on the espionage going on between the American and Soviet forces, but there are other topics within that time range (1946-1991) that could make for good settings. The Vietnam War would be the obvious choice, but Call of Duty could also break from convention and pick another conflict.
For instance, they could explore the Korean War, probably best known through its depiction in M*A*S*H. Like Vietnam, the Korean War was a result of Cold War politics that saw the country divided into a Civil War between capitalist and communist-supporting factions which got worse when both sides received backing from other countries.
A Korean War setting would be a good middle ground between the WW2 era of the older titles and the 1960s timeframe of Black Ops, and would probably have a lot of room to tie into other games in the series. Seeing as would only be five-eight years later (depending on the exact year the campaign starts), they could probably bring back some characters from Vanguard or, alternatively, tie it into Black Ops. Maybe there could be an appearance by a young Alex Mason or Frank Woods, or they could even throw in Captain Price's grandfather if they wanted a connection to Modern Warfare.
1 Vietnam War (1955-1975)
Call of Duty is hardly above dropping references and inside jokes to iconic Vietnam War films, especially Apocalypse Now. They even took inspiration from the film's depiction of Colonel Kurtz when designing General Barkov in the Modern Warfare reboot. However, the war itself has barely been touched.
Black Ops takes place around the same time and features multiple levels set in Vietnam. It would not be until Black Ops Cold War, ten years later, that Vietnam was revisited in the mission "Operation: Fracture Jaw." Both do a decent job, but there is only so much that can be done in a small stretch of gameplay, especially when the player is only there because of a larger problem.
The idea of a Vietnam-based Call of Duty game has been considered before. Back in 2010, Sledgehammer Games experimented with such a concept that would have made use of a third-person perspective. Unfortunately, internal problems led to the development team having to cancel the game, so they could help finish Modern Warfare 3, but there is definitely potential.
The obvious approach would be to focus on American soldiers, but there are some interesting ways the developers could take advantage of the setting and Call of Duty's popularity to introduce some new perspectives to the mainstream. One way they could do this would be to have sections dedicated to, and maybe even a playable character in, the South Vietnamese Army that was allied with the United States.
But if Activision is feeling more ambitious, they could go one step further by having portions of the game centered on the Viet Cong. With the moral ambiguity of Modern Warfare and Black Ops Cold War, it would be fitting to see a Call of Duty campaign where the player alternates between both sides of a conflict.
Imagine playing a level on one side and a character you've grown really invested in being shot, then the perspective suddenly shifts to the other side and reveals the shot was fired by...you.