Possibly in collaboration with an external developer.
Microsoft knows Halo is its big cash cow, and they’re not about to stop taking advantage of that fact any time soon. Even though we are already getting Halo: The Master Chief Collection (which contains HD ports of all four numbered Halo games), a Halo TV series by Ridley Scott, and Halo 5: Guardian within the span of the next eighteen months, it appears that Microsoft is hard at work on yet another game in the Halo franchise- this time with an external developer.
A new job ad was posted by Microsoft for a ‘new project’ within the Halo universe, in conjunction with internal and external teams. Now while that sounds like it could be anything- hell, it could even be a Halo manga, for all we know- I am personally holding out in hope for the long rumored and awaited Halo Kart. You know you want to race other SPARTANs in their Ghosts and Warthogs.