I had an idea. Have a moral/karma/reputation meter for the survivors. If theyre going around pillaging people, its not good, and since there's radio now, people know a fair bit of what youve been doing.
Bounties were a start, but try something longer-term and more well-known so people dont get to clear their reputation once a bounty is gone.
This changes things so players are actually encouraged to interact with other players/survivors.
You can help out a human compound, raid other people, or just kill zombies.
if you get a postive/negative reputation you could change it over time. An"angel" could become a "devil" by raiding alot of people, and vice versa by witholding raids and helping others.