Apparently, it has just been announced that Neversoft will now be helping with the development of the Call of Duty franchise. Activision hasn't given specifics yet, so they could be working on a new installment all together, or be providing supporting content like DLC or fixes to the games engine.
Neversoft has been looking for some new employees over the past year, with postings looking for people willing to help develop a First Person Shooter title. One of the key listing was looking for someone with experience working with PC's, and with a strong understanding of Steam distribution and security.
I'd greatly enjoy another company making Call of Duty games, even if the other two keep at it. Doing so would increase the production time for everyone while still keeping CoD's yearly cycle.