Raven Software was at one time working on a standalone live-service version of Call of Duty's popular Zombies mode, ex-Raven Software lead designer Michael Gummelt has revealed, but it was cancelled after its IP originator - presumably Treyarch - "wanted it back".
That small revelation comes via Gummelt's LinkedIn page (as spotted by MP1ST), which explains he was lead designer on an "ambitious new Call of Duty: Zombies live service project" between 2011 and 2012, but that the whole thing was canceled "when the [Activision] studio that 'owned' that part of the Call of Duty IP wanted it back."
Gummelt doesn't explicitly state the name of the studio, but it seems reasonable to assume he's referring to developer Treyarch, who created the mode for 2008's Call of Duty: World at War.