In most "Zombie Apocalypse" type games, shooting zombies is the only measure of skill. Not here...far from it, in fact.
When you log in, you can create a character. Perhaps a few strategic customization options here - select a policeman and you'll start off with a gun. Then, you are thrown into the game in a random location. The game world would be hundreds if not thousands of blocks across. Each "block" contains one place of interest, IE a police station, or a hospital, or a grocery store. You have a very simple interface holding the following:
This shows where you are and the locations in 8 directions around you.
This shows you your stamina and any other pertinent information about you.
This is where all your items are. You can use them (shoot, eat, bandage, whatever) or throw them away. That's about it.
This is what's in you area. All the items there (which you can pick up), as well as the "Fortification" level.
Now, the most important thing about this game is that there's basically only one key value: your stamina. Moving to another location, or fortifying your location, loses stamina. Eating gains it. If a zombie enters your location, you die instantly. If you run out of stamina, you die instantly. Dying causes all your items to be dropped on the ground, and you become a zombie.
Zombies cannot hold items. All they can do is walk around. Attempting to walk into a fortified area sacrifices stamina, but breaks down the fortification a bit. If a player shoots at you, you lose stamina. Either dying or killing a player will end this period and allow you to become a human again.
And that's really all there is to it. Most of these ideas have been rather open-ended, but the whole point of the game is that it's very, very simple. But these systems should self-regulate themselves, as long as you have a few active players then it could easily go on forever.