Killzone: Shadow Fall will get a co-op expansion in June, titled "Intercept." It will place you and up to three friends on Intelligence Squad Alpha, a special VSA team charged with going behind enemy lines and relaying information on the Helghast military.
The plot sounds like a loose framework to build a horde mode. You'll need to keep the Helghast at bay as you establish a hacking uplink, according to the announcement. You can choose from four combat roles: Assault, Marksman, Medic, and Tactician. As you take down Helghast you earn higher Team Scores, and you can "bank" your personal score to add to it. You'll need to protect your uplinks from assault, and that task is made trickier by the Helghast calling in special forces familiar to those who have played through the campaign.
It will come with four arena maps based on single-player locations, and will be free for Season Pass holders. A standalone version will also come later this year, for those who don't already have Shadow Fall but want to try the co-op.