Alright, here's my idea, which I entered as a blog and in the fake game friday competition. I like the idea of sim city, and I also like zombies. So what if they were combined? I'd think that would be rather fun. So here's the idea:
You would manage your city and such until you deemed it fit to start the infection. At the beginning of the game, you would determine : A. What percentage of your people would own guns, B. How intelligent your citizens are (which would determine how many get infected, where they go, what they do, etc.), and C. how many of them would be prepared (Do they already have a place to go and other paranoid stuff like that). There could probably be other things, but those are the main things.
Once you deem it fit for infection to come upon your city, you decide: A. Where the infection starts, B. how many Alpha Zombies(the first ones) to place around your city, and C. How long it will take for help (SWAT, army, etc.) to arrive. Once the infection starts, no more building can take place. Instead, you get to watch what happens, create safehouses in pre-existing buildings (The AI will automatically do this, but you can make your own), and control the police and such. You can build barricades in different places, and overall determine what your law-enforcement officers should do to survive against the zombies. Once help arrives, the infection ends, and you watch as they cut down the zombie masses. If you want, then you can rebuild, improve, and then start another infection.
All in all, I like the idea, but I'd like to know what others think. It's not a stellar idea, but rather a twist on a pre-existing concept.
EDIT: Also, during the infection, you can give up your city-overview status to become a person with a weapon of your choice. At this point, the game would become like a first-person shooter. I forgot to include this, so I added it.
Thanks for the comments, helping me to refine this idea!