Basically as you see in the end cutscene it is Samantha and Eddie as children in a bedrom. In the room you see multiple props from the zombies maps and games, including Perk bottles, Zombie figures and also a mystery box! (Which is where the props are supposedly kept). This is hinting to us that the zombie story line is just a figment of Samantha and Eddie's imagination. At one point Eddie says 'I didn't get my turn' and Samantha says 'You will get your turn next time!' this is hinting towards either another zombies map/game or a metaphor of how Samnatha controls the zombies on one map and Edward (Maxis) controls them on another, however it does not explain Richtofen. Also at the end you hear a voice shout to the children which is believed to be Samantha's dad and it sounds like Richtofen so this may explain it. Anyway all in all it makes us question wether the whole Zombies storyline is real or just fiction. But at the same time the ending could also be how zombies starts as Samantha say's 'My father has a plan' but yet again this could be hinting towards another map/game.