Two Ideas to be possibly used
Time travel in a zombies map would be awesome, players could start in say, WWII and travel to modern day, and also time travel would explain the use of some of the wonder weapons.
Regarding the # of maps to be used, I would be happy if Treyarch took a Mario Kart approach, where there are a few new and old maps of equal number out of the box or unlockable.
One more thing, make different mystery boxes for different time periods, so one would have World at War weapons, another Black Ops weapons, etc.
