According to a new job advert for 343 industries on the Xbox website, there's a new "Halo project" in the works at the studio, and they're hiring a producer to work on it.
The advert was posted over the weekend and lists the new role, based in Redmond, Washington, as a Producer role to help "develop a new project in the Halo universe," although the listing falls short of going into anything more specific. It's a pretty sure-fire hint that something is in the works though, but as for what that is, there are two possibilities to my mind;
While it's unlikely to relate to Halo Infinite, as that game is way too far into its development cycle at this point, it may be related to a future update or DLC. It's been said before that Halo Infinite is intended to be a platform for the franchise, with an extensive post-launch roadmap, so it seems consieveable that 343 may be looking to start up new projects to develop future content for the game.
The listing may also be referring to a totally new game or spin-off within the Halo franchise. Right now, it's impossible to say.
Halo Infinite is the next game to release in the franchise though, and is currently set to release in the fall of this year, after being delayed due to player feedback over the game's graphical fidelity.