Sledgehammer Games will take care of the rest.
Rick Grimes from The Walking Dead in Modern Warfare Zombies.

Treyarch is moving on from development of Call of Duty Zombies for Modern Warfare as the studio sets sights on future game development. Speaking with CharlieIntel, Treyarch says that they are moving "focus to something exciting on the horizon." That sure sounds like they are about to really buckle down on working on this year's Call of Duty, which is presumed to be Black Ops Gulf War.

New content for the Zombies mode has trickled off heavily in recent months. Today's Season 3 update for Modern Warfare 3 includes a bunch of new stuff for the Battle Royale and Multiplayer modes, but very little for Zombies. Though Sledgehammer Games will finish development on the remaining Modern Warfare Zombies story, I maybe wouldn't expect there to be too much more coming in the months ahead.

Fans of the mode apparently said the Zombies mode was dead already back in February. It seems as though Activision is putting this open world Zombies mode out to die already, much as they did with the DMZ mode with Warzone 2.0.