There was a lot of backlash when Activision announced that Call of Duty: Infinite Warfare was going to the far future for its adventure. Those who are sick of the focus on sci-fi were hoping the series would go back to its roots in the past, but alas, it did not.
However, according to IGN Spain, we might be going to Vietnam next. They are reporting rumours that Sledgehammer Games' next game will take us back to the 60s, which would be fitting, as it was reported that the studio was working on the setting before deciding to develop Advanced Warfare instead.
A trip to Vietnam would suit Call of Duty, but with the war still in the memories of some, this would be something Activision would have to be careful with. We are seeing this with Battlefield 1, with tough eyes on the way it depicts WWI. I assume Vietnam would be even tougher with many involved still alive today,