My first idea is to add deterioration. For example: Houses slowly tearing away, tiles falling into the earth, etc etc.. You could survive for MONTHS (in-game) on end and the surroundings would still be the same! First example: Buildings. When building deteriorate, so will the structural integrity. Which means buildings will collapse. Effect from this are: Noise. The noise radius will be slightly larger than an alarm. SLEEP TIGHT! Any walkers in the house when it collapses will either be killed (again) or will turn into crippled zombies. Second example: Pathways. Nothing lasts forever folks! Well, maybe honey. But that's not the case. Dirt paths and roads and pathways and others will slowly rust away. Meaning that it will be harder to locate certain areas. No paths=uncertain travels. Just, hope that you wont find the mall.