Atlantis would be full of tunnels, like Moon. And if you wore a suit, like in Moon, you could go into flooded areas, where gravity wouldn't work and you could float around, like Moon. Atlantis could be an abandoned underwater military base or something, so there would still be a lot of indoor space. It would also follow the trend of BOII maps being related to BO1 maps.
Die Rise - Ascension
Mob of the Dead - Call of the Dead
Buried - Shangri La
Atlantis - Moon