my cousin and i have both said that since BO1 and ive seen many people say it on here. the thing is i'm not sure it will ever happen. Because activision knows it can get a bigger amount of people to buy the whole thing rather than two cheaper parts. i feel that if zombies did develop into its own game it would either have to be cheaper standalone or purely digital with say the 3 starting maps and then the other maps ranging in price depending on how popular they were/are or what game they're in. (older ones would most likely be cheaper due to age and what they offer but they are more popular so deciding on a reasonable price would take time)
you know what screw all that, just put all the maps on one disk