User blog comment:Argorrath/Treyarch posts a message of zombies; possibly hinting to a DLC 4/@comment-1263839-20130801161910

Here's to hoping that DLC offers some damn closure, I mean proper closure not a cliffhanger in a fancy dress.

Also, as RGS has proven Treyarch always appeal to the masses (Samantha) rather than what makes sense so chances are it's the original characters writing these letters.