I think it would be fun to find the bodies of other “survivors” occasionally, with a journal that gives some hints as to where they’ve been and what they’ve been doing. They could even offer information about the world - i.e. “Day 4: I woke up to find the cabin surrounded by triffids again. I’m heading west to get out of their territory. Day 5: I came across a creepy motel and have decided to use it for shelter. - End of Journal”. Examing the corpse should help you identify the cause of death, like “mauled by a bear”, or “covered in hundreds of squirrel bites”.
It would be nice to uncover personal stories like that, which you don’t get with the current NPCs. Players might want to give them a decent burial or build up a library of their personal accounts. Maybe their bodies could hold unique items, and graffiti could provide a paper trail for players to follow (like a treasure hunt). I think it would make the game world seem more alive. Er, even though everyone’s dead. (You know what I mean.)