First of all, I am using translator, sorry for any possible errors in the text.
This idea is mainly thought for Servers since in SP I wouldn't know how to approach it but maybe you can help me.
My idea is to create a currency that you can trade with other players to get resources and what is that currency?
Zombie Ears
It seems to me a great balanced incentive that for each Zed eliminated you can cut the ear with some knife and thus be able to trade them for other resources. That's why I said that in MP it's easier to manage this but in SP I wouldn't know how to do it, maybe interacting with a radio and exchanging the ears for resources...I don't know.
I hope for your help and someone who thinks it's a good idea and decides to create it, I could provide the textures.