Since 2012, Activision designed a three-year cycle of work for each new Call of Duty. Treyarch, Infinity Ward and Sledgehammer rotate as the studios in charge for Call of Duty. Call of Duty 2019 will be the turn for Infinity Ward and possibly their new Modern Warfare. Next year, Sledgehammer will take the lead, but reportedly, that is not going to happen.
Jason Schreider, has published an article about the current state of Call of Duty 2020. The game is in turmoil. Activision has decided that Treyarch will take the lead for the game and Raven and Sledgehammer will serve as support studios for the game. Treyarch will release a new Black Ops for 2020 and the work done in the campaign for Sledgehammer will be used in Black Ops 5. The game would be set during the Cold War (Vietnam could be in). Likely, Call of Duty 2020 will be a cross-title for current systems and new machines.
The last, but not the least, Activision could have been evaluating to introduce free-to-play as a way to get revenue in Call of Duty 2019. Free-to-play as a component inside the title environment. Some at Activision would not support that idea.