The 'Letters from the Field' images that Treyarch have been uploading to their Facebook page enforces this theory as they are all dated in 1917. (Before the Zombie outbreak).

My theory is that you will play as the 4 original characters and it will be like a prequel before Nacht der Untoten. (Or Der Riese if that map is set before Nacht). Also, i'm guessing the beginning cutscene will show you how the zombie outbreak begun and how the four protagonists must work together against their will. I can imagine that all of them will hate each other and do not like working together. Finally I believe that once you do the easter egg, the 4 main characters minds will be erased, like it is now. Leading up to Nacht der Untoten or Der Riese.

Tell me what you think please.

I accidentally put this in the Black Ops 1 forums. If someone can delete it, thank you.