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A job post has revealed that Microsoft is working on their own Mixed Reality game for the PC and Xbox.
The title will presumably support their new range of Mixed Reality headsets from the likes of Acer and HP, and according to the job post will be a large multi-player game which we assume will support cross-platform play.
https://mspoweruser.com/microsoft-creating-massive-mixed-reality-multi-player-online-game-xbox-pc/