just differentiate the loot so that in bigger ,more fancies cities you'll find better loot,but change the zombie ratio so that in these cities there will be A LOT more zombie that in less loot-profitable cities/places. so even if you can't tell if someone is hostile or not,people who want better loot will be forced to cooperate with other survivors even if the usual reaction would be "shot first and ask later",while if someone wants to be a lone wolf (being a lone survivor or a lone PK/bandit),as chances of surviving just staying away from the big cities,but with worse/scarcer loot. things can be interesting when you go to a big city with a big group trying to get big loot,and people begin betray each other for the more interesting loot,while being aware that less people around means more zombie-to-kill ratio for anyone.
