User blog comment:Argorrath/Treyarch posts a message of zombies; possibly hinting to a DLC 4/@comment-213.107.179.220-20130731143029

Crackpot theory time: This is actually written by Richtofen with 'Doctor' referring to Maxis. The 'oldest friend' is a new character who is killed/driven insane by element 115 and so Richofen's ultimate goal has actually been to save his friend this entire time. You play as Maxis, Richtofen, the friend and another scientist in the map and the EE is similar to MotD (having you kill the friend)

On a more realistic note, I would expect a huge cliffhanger like Moon but the way Buried played out makes it seem that this would be entirely seperate from the current storyline (Buried being the cliffhanger I mean). If it was an origins story of sorts, then the zombies would most likely once again have the red eyes so heres another crackpot theory: the events of MotD allow Arlington to survive and come into the current timeline hence including him in the storyline (perhaps to replace a character that could be killed off because thats bound to happen at some point)