Basically it's a lot of overwriting data. Hopefully collisions should never happen, since the map is big enough that the chance of two players being in the same place at the same time is miniscule. And even if it did happen, it'd just result in some minorly nonsensical disappearance of items.
Could you make it into an MMO, or even multiplayer, with any amount of code? I'd pay to hell and back to play a zombie survival roguelike MMO. I'd imagine intermediate changes to the framework, the most major foreseeable being adding a standard unit of time for a "Turn". Have sessions end/world restart when certain conditions are met or a certain number of people have died. It already has all the standard fare of MMO games (Raiding hives, underground labs, random encounters, clearing out sewer tunnels) but more stuff would have to be kept track of, and item spawning would have to be added to some degree.
Not to mention Putty is a great way of weeding out griefers. It'd be really fun, all sorted. Just require more balance.
Forgive me if my request seems stupid. Maybe I'm just excited.
I've been thinking about the possibility for a long time now.
First off, there's adapting a roguelike to simultaneous action. I suppose I could limit each player to a couple seconds to take their "turn," but no matter how this is done I feel like it'd be slow and obnoxious.
And then there's technical issues; how do I use inter-process communication to coordinate everything? How do I guarantee that data files are locked, and how do I handle a player attempting to access a locked data file? And even once I figure out good answers to these questions, I have to learn how to code it all.
I think direct player/player interaction will have to wait until 2016, when Cataclysm: The FPS is released.
hnnngh.
You could always find someone to help you code, unless you feel Cataclysm is too much your kid to bring in a second parent, as it's usually put. A few seconds isn't really required, but what you could do is give the zombies number, instead of speed. Fighting ten zombies/running from twenty zombies is a lot harder and exciting than trying to flee from a zombie that can just BARELY not-keep-pace with you. Pacing isn't really an issue past seeing what the users feel is balanced, anyway. Locking data files would require a proper login system inside the actual game client itself, and even if something were hacked, data files would only last until the end of a session which, if steamrolled, would only last for a day or two before being deleted and reset anyway.
It'd work itself out, you just have to find a way to properly approach it. And you have a fanbase - of roguelike players, mind you - certainly one of them could help you.
Honestly. If someone wanted this to have real multiplayer I think it would work as a MUD. Not so much as a growing of the main project as making it into a new branch sorta. It would likely need to be completely re-coded but that's the only way I see it working myself.
But it already has everything any MMO would have. A massive map, dungeons of varying difficulty (From doom to super-doom, they practically already require parties) and even an overlapping userbase. I'm sure you've seen a few dozen zombie corpses that you didn't make, if you've played for more than an hour on Putty, so some of the interactions are already there.
The only way this wouldn't happen is if Whales didn't want to take it there.