Maybe it would be possible to just start with a few "weak" NPCs in each house, and no zombies until day 3-5? Then the zombies will ramp up the spawn rate over 2-3 days to simulate an outbreak, while possibly killing off the weak NPCs as they start roaming around. It would be easy to imagine the outbreak already being known at this time, but the spread has not reach our lovely little town yet, as to not break immersion when raiding random houses in preparation of an invasion. No military, no tv/radio reports, just a ramp up period to give the player a head start and different experience. I, for one, would love to see this as an option in the future, as getting a bit more prepared before the onslaught would be interesting.