Possibility a band with a few NPCs or join an existing one. Intergroup conflicts over territory, looking for food, trade, joint killing zombies, construction camps and other community activities. Joint drinking and other. (“Who took my shotgun, basterds?” For example)
I know that it will be difficult to implement, but it would not be bad.
This sounds like a great idea. Currently the idea is very similar to the “faction” idea of NPC’s (which isn’t implemented that well), but I could certainly see smaller NPC “gangs” or such-like forming or dissolving as well.