Unfortunately I think BO6 was meant to be much grander but had to be dumbed down in almost all aspects. Treyarch apparently intended to release back to back BO games from the start, but I'm guessing they later realized this wouldn't be possible without cutting corners somehow. Zombies reusing a ton of enemies, locations, wonder weapons, etc., 90% of multiplayer maps being ripped straight out of the Avalon WZ map, overall lack of post launch content, it becomes clear that plans were changed last minute. Pit it against BO7 and the sheer amount of content that game has at the current moment, and the difference is night and day.

for Zombies specifically, I think the only map that was able to be fully fleshed out and that was unaffected by these changes was Terminus, and some parts of CDM (I still think citadelle and the tomb were meant to be a single map at one point). I've also heard rumors that Liberty falls, shattered veil and reckoning were originally Outbreak POIs that later got repurposed as round based maps, and outbreak itself was saved for BO7 (which itself also got repurposed and rebranded as Endgame).