Given that last year's Call of Duty was developed by Treyarch and Sledgehammer Games' take on the franchise, titled Call of Duty: WWII, was released in 2017, it's safe to say that Call of Duty 2019 is being made at Infinity Ward, the makers of COD: Infinite Warfare, Ghosts, and the Modern Warfare trilogy.
Rumor has it that this year's COD is a full-blown sequel to Modern Warfare 3, bringing the series back to its roots with the inclusion of a proper single-player campaign and boots-on-the-ground gameplay.
In the upcoming game’s Narrative Director Taylor Kurosaki's opinion, COD 2019 will be an "ambitious" title, but it's unclear what it actually means. Kurosaki used to be an employee at Naughty Dog (he had an opportunity to work on the original Crash Bandicoot, Jak 3, and Uncharted: Drake's Fortune), but he left the Santa Monica-based developer to help Infinity Ward with their highly divisive Infinite Warfare.
Considering that COD: Black Ops IV was officially announced in May, there's a chance that Activision will reveal new details on Modern Warfare 4 in the coming weeks, right before the start of E3 2019.