If the established developer rotation for each Call of Duty continues in 2019, then Infinity Ward is next. Infinity Ward's senior communication manager Ashton Williams send out a tweet back in January that featured a skull, which may hint that it's Infinity Ward at the helm this year and that the next Call of Duty may be Modern Warfare 4.
Since Infinity Ward is the developer behind Call of Duty: Ghosts, many first suspected the tweet was teasing Ghosts 2. However, Kotaku's Jason Schreier has since claimed in a forum post that isn't the case. Prior to Ghosts and Infinite Warfare, Infinity Ward was responsible for the widely popular Modern Warfare series. Fans may recall that one of the major characters from Modern Warfare 2, Lieutenant Simon "Ghost" Riley, wore a skull-patterned balaclava. Williams' tweet may be in reference to that, which could mean there's a Modern Warfare sequel starring the character scheduled for 2019.
However, considering the popularity of Modern Warfare Remastered, Williams tweet could instead just as easily be a hint that a remaster for Modern Warfare 2 is in the works as well. Infinity Ward released Modern Warfare Remastered alongside Infinite Warfare, so the developer could be planning to do something similar with whatever new game is planned. This is all conjecture though, and the tweet could just as easily be a tease for a brand-new Call of Duty series that has nothing to do with Modern Warfare.
Treyarch's 2018 Call of Duty: Black Ops 4 was the first game in the franchise to not feature a traditional single-player campaign. Instead, Black Ops 4 included the franchise's first battle royale mode, titled Blackout. However, this year's Call of Duty will see the return of a single-player campaign.