You're completely right, there has to be an incentive for players to want to go out of their way to get; the reward has to match the risk. Perhaps in high areas of zombie population would spawn higher valued items, weapons or vehicles. Also, clearing zombies in these cities may increase humanity. Whilst humanity currently gives you the hero skin, in the future it may lead to say... being able to give a transfusion slightly quicker, bandaging quicker etc. The benefits can't be too overpowered, but worth the good deed. Without turning this into WoW, having areas that simply can't be looted by new players would result in more teamwork between serious players, and end game content.