Whether or not the word 'Vanguard' features in the final title, it seems increasingly likely that we'll be returning to World War Two. Sledgehammer was the lead developer behind 2017's Call of Duty: WWII, so could easily be set to return to that period. What's more, a
Call of Duty leaker has already suggested that this year's game will be set in World War II, and ModernWarzone also reports that some of the game's cutscenes will be set in the 1950s, soon after the period in which the war took place.
Activision has confirmed that this year's entry in its FPS series will be coming in late 2021, as is pretty much custom at this point, but we've had very little in the way of official information since then. That said, Call of Duty's reveal events tend to be in the summer, so we should find out more relatively soon.