Just thought I'd point it out, at no point was the number 5 mentioned at all...
I'm betting it'll be a remastering of the Xbox/Xbox 360 Halo games for Xbox One, with a port of Halo 4's multiplayer, it makes more sense really.
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I'm pretty sure Microsoft has been on the record of saying they want to make new Halo games every year. Meaning there will be standalone games or something that is released during a year in which an official Halo game is not released. My guess is this Halo game is not Halo 5.