Next game in the Call of Duty series is currently rumored to be Modern Warfare 4 based on a supposed leak during a recent Call of Duty co-promoted event held by Activision and the NFL.
Also fueling speculation of this game being Modern Warfare 4 was a report from Eurogamer about this event that sneaked in the game's title via a coded message trick.
https://www.eurogamer.net/articles/2...have-played-it
https://www.destructoid.com/the-new-...4-551329.phtml