According to a job listing for a Gameplay Designer on Halo, 343 is looking for a "passionate Gameplay Designer that can contribute and implement exciting gameplay moments into a next-gen Halo Shooter."
This job listing seems to be referencing hand-crafted moments, like set-pieces possibly hinting that this next Halo game would be single-play or at least have a campaign component.
Whilst you may be thinking this job listing refers to Halo Infinite or some kind of DLC, the website actually has another job listing for a Character Technical Artist, where they specifically say the job is for Halo Infinite.
So, it does seem like this is a new project of some kind, but the job listing doesn't offer any more details unfortunately.
This could be an entirely new project or it could be referencing a DLC for Halo Infinite, maybe even a standalone small experience. We just don't really know. However, we will keep on eye on 343 to see what they are up to.
Meanwhile, Halo Infinite will headline Xbox's E3 showcase and you can keep up to date with everything we see from it at the show by heading to our E3 2021 hub.