I like the vermin system, leave rotting stuff around, and bad things grow from it. However, it ought to be expanded more. Leave food out in the open or not in a container, and mice or other vermin can start spawning from it like the roaches. Granted this would have to be done carefully, or scavenging for food might become a nightmare if everywhere you went were vermin nests instead of foods. And if you get vermin nesting in your walls, maybe you can start finding mouse holes in walls or something. A step beyond that, if you ignore vermin too long, they either grow or start attracting predators to the area.
Another thing this game doesn’t really have is little pesky thieves. What if there gremlin like creatures or mutated racoons that try to take your things. If character is light sleeper could wake up to noises coming storage area, and find bunch of little things jacking your stuff you forgot to put away. Not trolling levels of theft, just pilfer a couple of things here and there. Maybe one day I go to fight a horde around the corner, and notice my weapon missing from pack? Perhaps the lil’Btards steal from zeds & npcs as well getting reactions out of them. I would be fond of the idea of searching my basement for missing things to find a random burrow in wall that leads to a nest piled high of my & whoever else’s loot.
This idea mostly comes from the fact that I noticed there really aren’t any interesting passive mobs in the game that do anything much or rather don’t impact your gameplay. Could also apply this idea towards other mobs like for example beavers. Beavers could slowly knocked down trees in area, chew up your wooden stuff, and begin damming up rivers. Although i’m biased towards having a trained pet gremlin that i could send out to steal stuff for me, then screw myself by feeding him after midnight pool party.