I would also make a game based off an amazing flash game I had the pleasure of playing on Newgrounds
here.
I recommend it be played for a full explanation of what exactly I'm talking about.
Basically however you would be required to clear zombies from sections of the city and then barricade that section of the city off. There are survivors, different types of city, morale you have to manage, and random encounters. There are a ton more features I would add but 'nuff said I guess.