For well over a decade, Activision established a pattern of announcing the next Call of Duty game by May of every year. However, that changed with Call of Duty: Black Ops Cold War, which wasn't announced until late in the summer of 2020 and released a few months later. It seems that Activision is doing something similar with the 2021 Call of Duty game, which is now expected to be revealed at some point later this month.
The 2021 Call of Duty game, rumored to be entitled Call of Duty: Vanguard, is expected to be revealed through the Call of Duty: Warzone battle royale. As it so happens, the Call of Duty: Warzone battle royale is heading into Season 5 of its Black Ops Cold War crossover on August 12. It's quite possible that Season 5 will kick off the Call of Duty 2021 reveal, but it could also happen a bit later in the season.
The incredible popularity of Call of Duty: Warzone seems to be what's fueled Activision's shift in Call of Duty game reveals. Considering this, it seems Call of Duty fans can expect game reveals happening in the late summer to be the "new normal" for the franchise.
There are rumors that Call of Duty 2021 will have even more significant Warzone integration than Black Ops Cold War. According to the rumors, a brand new Call of Duty: Warzone map based on Vanguard is in development and will be available either later this year or early next year, depending on who's leaking the info. Apparently, Call of Duty: Warzone's new map will be World War 2 themed and comparable in size to Verdansk, as opposed to being a small-scale map like Rebirth Island.
Again, none of this has been confirmed at the time of this writing and so fans should keep their expectations in check. If the Call of Duty: Vanguard reveal does happen this month as expected, though, then fans should have concrete details sooner rather than later.
Of course, this is assuming that Activision's marketing plans for Call of Duty 2021 haven't been completely derailed by the ongoing conflict between the company and its employees. The state of California is suing Activision Blizzard for gender discrimination, and that has resulted in some severe backlash from employees and fans alike.
Call of Duty is in development for unspecified platforms.