Contrary to popular belief, Call of Duty has evolved over the years. It's just that we get a new entry so often that it feels like every installment is the same as the previous ones (they aren't). And, while the most recent installments haven't been too original, Activision Blizzard does indeed put a lot of effort into making each game, employing roughly a dozen studios to work on the franchise exclusively. This includes Raven Software, which, to most, is a name associated with Call of Duty: Warzone. But, as it turns out, there was a time when the Wisconsin-based studio could've worked on a standalone Call of Duty: Zombies game.
Apparently, Activision once explored the idea of a live-service game centered around zombies. This project, which never came to fruition, was in development as early as 2011-2012, showcasing the forward-thinking approach of the developers and the potential for expanding the Call of Duty universe in unique ways.
The Zombies mode first made its appearance in 2008's Call of Duty: World at War, developed by Treyarch, and rapidly gained a dedicated fan base. Its blend of survival horror elements with the first-person shooter mechanics of Call of Duty struck a chord with players, leading to its inclusion in subsequent entries of the series. The mode's popularity is something that has always fueled discussions and desires for a standalone game, something that Activision has yet to explore seriously, at least publicly.