Mine is what idol said. I had a huge thread about it before with all my ideas. Dunno' what happened to it. The main thing would be NPCs that you could rescue who would perform actual functions, so you can play it safe and keep them at the home base, or you could send them out in the hopes of them finding more gear. Some would be researchers, some would be brawlers, etc. The smart guys could eventually come up with better designs for defenses and such, but you'd have to make sure that you defended them from the zombies who try to invade. The zombies would have a "flock" AI system where they'd be a piece of cake alone, but lone zombies could eventually lock into a bigger flock, and bigger flocks would pose a huge threat to your home base without adequate defense both structurally (traps, wires, boards, spikes, whatever) and man-wise (big guys with pipes and guns, etc.). I think I'd have some kind of thing so that lone zombies take more damage and are harder to kill, yet they pose basically no health-threat to the player, but flock zombies are the opposite. They die really fast, but they dish out huge damage and move faster. So if you run across a lone zombie, it's like... do I waste extra ammo and kill it, or is it far enough away from the known flocks (which would be only moderately difficult to track) that I can save the ammo and not kill it?
But yeah, I've been wanting this game for years. Dead Rising came really close in a lot of ways, but it wasn't a full city and the NPCs are just there to be rescued. I want it to be that the player is in charge, but he essentially builds a team of survivors through the scenario and has to use risky play styles in order to get new gear... so that taking more risk means greater reward, but also has the chance of cutting you off from the home base if a flock of zombies gets too close... and playing it safe means you'll be able to keep better tabs on the base, but you wont' be able to get much in the way of new gear and stuff.