Microsoft seems to have a busy year ahead with Forza Horizon 6, Fable, Gears of War: E-Day, and Halo: Campaign Evolved, but they obviously have many more projects in the works, not least those that have been announced. It seems that one of them may be a slightly different title, namely a military-themed horror game.
This was revealed by a former Xbox Game Studios employee via his LinkedIn profile (thanks Tech4Gamers), where among the games he has been involved in, we find an "Unannounced Sci-Fi/Military/Horror First-Person-Shooter," on which he worked as Sr. Narrative Director. It is listed as a game he worked on after Microsoft Flight Simulator (released in 2020), so it seems to be a fairly recent game, which should not be confused with Gears of War: E-Day as it is not first-person, while Halo: Campaign Evolved can hardly be called horror-themed.
We don't know who is developing it, but studios such as MachineGames, Id Software, Treyarch, and The Coalition come to mind. If any of these are involved, we'll find out when/if the game is actually announced.
We can also add that the same person lists that he has worked on an "Unannounced Open-World Fantasy RPG" at Xbox Game Studios as well. This project is also unknown, and studios such as Bethesda, Blizzard, Ninja Theory, and Obsidian Entertainment would all be good fits.
We'll keep our fingers crossed that one of them will be presented at one of this summer's many events, but until then, we'll just have to wait and see.