When the NPC's arrive in the game, that feeling of complete lonelyness will disappear, as players will know they are not the only survivors from the knox event in the area (By that I mean in singleplayer). I believe that players should be able to turn them on/off when crating a new save or server.
I also would like to config the NPC's in our games. (e.g. maximum number, etc)
my other ideas:
NPC's can spwan with/ join a group with other NPCs
NPCs can build safehouses, communities and more.
Kinds of NPCs, like nomad, scavenger, outdoorsman (live in the woods).
NPcs may arrive from outside the map borders during the game (affected by the max. quantities of npcs in the game)
Metagame events, like gunshots and the sound of a car crashing (another idea, can only ocour during the first 10 days) may spawn injured/uncounscious NPC's, who you can aid to gain their thrust.
Friendly NPCs can join players in their base (?)
bonus
In the animal update, maybe adding racoons is a good idea
maybe dogs, since there is a barking sound in the metagame events, may be with an NPC owner.