Z levels are pretty tricky in roguelikes in general. I remember it took a
looooooooong time for Toady to finally release a version of DF with a good Z-level system, and as far as I know it's the only roguelike that's done something similar. It'd be a pretty major undertaking without using workarounds and tricks.
Zombie Spawns: Make it somewhat customizable? Current system could be high difficulty or challenge mode. Default could be something more realistic. I also think the spawn rates are pretty ridiculous. They don't even come from anywhere. You make some noise, and they just start pouring in from all edges of your screen, even from areas that you just explored. It would make a lot of sense to me if the majority spawned from within nearby buildings, especially ones that you haven't explored. Then you have some time to move around and prepare after making noise before they're on top of you, and you can't just run through a nearby building and be guaranteed not to encounter anything inside that could foil your escape.
I dunno, even the way it is now is pretty easy for most veterans. There could be multiple options for spawns, where you can adjust the spawn rate (Hardly Anything to Don't Breathe Too Hard), population (Podunk Town to Megalopolis), and hoard chance (Hoardly Anything to The Streets Runneth Over).
The ability to tweak zombie perceptions (So they all don't automatically swarm your exact location, and occasionally wander aimlessly in the street) would be nice, too, but that'd require a bit of AI work. Dropping zombie perception to the minimum and jacking population and spawn chance to the max would be a trip.