"343 Industries is searching for a rock star engineer to join the team developing a new experience in the Halo universe," the job listing reads. "Work on one of the most exciting and creative intellectual properties in the industry; we are gathering an incredible team to help drive the future of Halo."
The emphasized part in the first sentence appears to confirm a separate title is in actively in development. Furthermore, this sentence confirms that this will be a AAA release and not an arcade or mobile release like Halo: Spartan Assault.
"We are looking for a talented software developer with the skills and drive to help us deliver the online and connected features of a AAA game title," the listing reads.
Aside from being a position with 343 Industries, the job listing is listed under the Devices and Studios Engineering which likely puts it on the Xbox where the vast majority of Halo releases have occurred. While the job listing does mention experience shipping a current generation AAA game title, that is likely due to the fact that the Xbox One is so new. There are few games in active development for the Xbox 360.
Non Master Chief Halo games that have been released so far include the real-time strategy title, Halo Wars; the side-stories, Halo 3: ODST and Halo: Reach; and the Windows 8 PC and mobile release that eventually made its way to the Xbox One, Halo: Spartan Assault.
Whatever the game ends up being, it likely won't make an appearance this year as Microsoft has confirmed that Halo 5 will be released to the Xbox One later this year.