While Call of Duty: WWII is no doubt the focal point of series fans everywhere, but behind the scenes, Treyarch is well underway making sure they are ready for their 2018 entry, whatever it may be. A recent job posting may give a little insight as to what the game might be or at least a potential era it could take place in. Treyarch is looking for a Combat Systems Designer, and one of the things they list as something that would really make a candidate stand out is a "deep knowledge base of firearms and modern military technology."
Here's a screengrab:
Having a knowledge of firearms is nothing out of the ordinary, but it's that "modern military technology" bit that pops out. This would seem to indicate that the 2018 Call of Duty won't be returning to a historical time frame and the series is only just starting to turn away from the futuristic settings of Infinite Warfare and Black Ops 3, so it seems unlikely that they would dive back in so quickly.
One other possibility is that of a Modern Warfare 2 Remastered Edition, though that would also seem odd since you'd think Infinity Ward would be better equipped to handle it.
Neither Activision nor Treyarch have publicly commented on the 2018 Call of Duty, nor do we expect them to with WWII so close to release. The new Call of Duty's aren't typically revealed until around April in the new calendar year, so don't expect anything concrete to come out until then.
Call of Duty: WWII releases on PS4, Xbox One, and PC on November 3rd, 2017.
Source: [Treyarch via Charlie Intel]