I have a whole 10-page document on how this would play out, the plot, and ways to make it fun while STILL trying to keep the original WAW-BO3 Zombie vibes.
I feel like Modern Zombies is stuck in a awkward position where they are trying to be modern yet, they cannot change fundamental core aspect of the game mode without backlash (Mystery Box, Perks, etc). This leaves us with a half-baked experience.
I truly believe that the best way to make a separate side-game mode that takes inspiration from other modern Survival Horror games (e.g. Resident Evil, Silent Hill f) and incorporate it into Zombies as a Endless Roguelite mode, while keeping the original concept of classic zombie for the main game-mode.