In the weeks leading up to the Call of Duty: Ghosts reveal, fans tended to use "Modern Warfare 4" as shorthand for the unnamed project. Apparently that title wasn't just for fans, as executive producer Mark Rubin says the game actually started as a direct sequel, before the studio changed the name.
When Infinity Ward was pitching new ideas, it threw out ideas ranging from post-apocalyptic to what Rubin playfully nicknames "Space Guns on Jupiter." The idea of familiarity was well-liked among the team, though.
"People felt really strongly that they liked the way you as a player can connect to the world you know day-to-day," Rubin told Game Informer "So the idea of staying modern became a key point. Let's not do 'Space Guns on Jupiter.' Let's do real weapons that we know in a world we're familiar with. And then it became, do we do Modern Warfare 4? And that was the game for a little while. Because we said, we'll stay modern, we'll do Modern Warfare 4! But then it was like, well, we kind of finished the story in Modern Warfare 3. That arc is done." This echoes statements the studio made at the time when finishing MW3.
From there, he said, the studio went back to considering a post-apocalyptic setting. Still in the mindset that it had to follow established continuity, they started to imagine that sometime after MW3, nuclear weapons had been set off around the world and decimated it. Regardless of what name was used, that would have more directly followed the events of the current series.
Ultimately, though, it came to the conclusion that with an entirely new engine, it could do somewhat of a reboot. As a result, Ghosts is set in the same time frame as the Modern Warfare series, but in an entirely different continuity. Rubin called this the "Earth 2" of Call of Duty, playing on the DC Comics book that features alternate versions of famed superheroes.