343 Industries is a huge developer with roughly 450 employees. They have the responsibility for taking care of all things Halo, and right now they are developing Halo Infinite. But with a studio this big, there is of course more than one team, and thanks to job listing, we now know they have at another Halo game in the making.
In the listing, we can read that 343 is specifically looking for "a Senior Producer to work with our Core Services team to help develop a new project in the Halo universe". As Halo Infinite probably has live service features and will be built upon continuously, we assume it's not a sequel. It could also be Halo Wars 3, but our bet would be on something completely new.
Whatever this "new project in the Halo universe" is, don't expect it to be revealed for at least a year or two.