With Call of Duty: Advanced Warfare, the Call of Duty series started to lean harder into science-fiction than past games in the series, introducing jetpacks as a new means of traversal in the game. Call of Duty: Black Ops 3 continued with the jetpack trend, as did Infinite Warfare, with the latter being set in space in the far future. Call of Duty has since gotten back to its more boots on the ground approach from the earlier games, and it doesn't seem as though the franchise will be returning to jetpacks anytime soon.
Treyarch developer David Vonderhaar replied to fans on Twitter to confirm that Call of Duty 2020 won't have jetpacks. Vonderhaar stopped short of revealing any other details about what Call of Duty 2020 will entail, but it seems to safe to assume that this means that the game will either have a modern or historical setting. There are a number of possibilities when it comes to what that setting could be, though rumors have pointed to Call of Duty 2020 being set during the Cold War.
More specifically, Call of Duty 2020 may be set during the Vietnam War. Rumors and leaks have pointed in that direction, and it wouldn't be the first time that a Treyarch-developed Call of Duty game was set in Vietnam. However, concrete details are unavailable at the time of this writing.
Traditionally, new Call of Duty games are unveiled in May, with further details revealed throughout the summer months. There doesn't seem to be any reason for Activision and its teams to diverge from that, though there may be some extenuating circumstances that sees things changed up a bit for Call of Duty 2020.
There have been reports that Call of Duty 2020 has had a troubled development cycle, apparently due to conflicts between Sledgehammer Games and Raven Software. These issues then saw Treyarch brought in to salvage the project as the lead development studio, with some reports indicating that Treyarch has turned the game into Black Ops 5. Activision has mostly kept this situation under wraps, though, so it's hard to say with certainty what exactly is happening with the next Call of Duty game.
More details on Call of Duty 2020 will likely be revealed this coming May.
Call of Duty 2020 is in development for unspecified platforms.