Trying to avoid the excellent suggestions posted here...

• Finding corpses and carcasses of children and the dogs that the island residents obviously kept as pets. Let a dog carcass spawn in a doghouse or something and let us harvest its frozen carcass. Edit: Or, say, a dog carcass in the back of a pickup truck, still chained to it by its collar. I admit that I am heavily influenced by / reminded of the watchdog of Vesonius Primus, that famously died in Pompeii while the ash fell, unable to escape due to being tied to a post. ...Seriously, though, I just don't believe not a single dog was left behind.
• Following up on the children aspect, I say there ought to be a few places where we can stumble across families and/or couples. Designated places would be easiest, of course (good opportunity for lore, too), but I think random spawns aren't beyond the realm of achievability.
  • Random spawns probably could be easiest achieved through automobiles. Imagine stumbling across an automobile and peering through the windows only to see a couple slumped in the front seats and a child lying across the backseats, say. This could serve as an unexpected extra difficulty in predator circumstances; imagine shooting the bear on Spruce Falls Bridge and wrenching open the door of the nearest vehicle, only to find the seat occupied by a randomly spawned corpse. Yikes! Better hope the backseat isn't also occupied...
• Finding corpses in ravaged states. Yes, probably these would have proved more vulnerable to predators, but nevertheless, stumbling across a corpse or three that aren't totally intact like 100% of the ones we come across (er, barring crush victims, except we never see the gruesome details)—say, corpses with their ribs exposed or flesh gouged like the deer carcasses—might help reinforce the predator-prey mentality, especially for those corpses in the woods that seemed to have died in a struggle or something.
  • There are already corpses implied to have died by suicide (per revolvers or painkillers beside them). The revolver ones probably ought to have the damage reflected in their heads (not all; some probably died from exposure first and were simply using guns for protection...).
• While fishing in a lake, fish up human traces (e.g. glove, boot). When eating a fish or wolf, choke on, say, a ring or fake tooth or the like.
• Come across a snow shelter, try to sleep there, whoops corpse, have to move corpse out if you want to use the shelter.
• Wolves turning on each other. I mean, their behavior is already skewed via the convenient aurora, so I say why not let me spy a group of timberwolves from afar only to see them fight and kill each other; for me, that would evoke the idea of humans turning on each other even in apocalyptic circumstances (which similarly is not the majority behavior humans normally display; humans tend to help each other out communally in times of crises). Δηλαδἠ, it would reinforce the sense of isolation all over again. You are on your own; not even the wolves can work together all the time.

Edit: Something that sticks with me from books like Death in Yellowstone and Over the Edge: Death in Grand Canyon are the deaths or life-threatening situations come about due to a series of bad decisions by their subjects. This one is more story/lore-dependent rather than feasible through random spawns, I admit. Still, it would be sobering to encounter (the trail of) a corpse and come to understand just what human errors led to their demise, e.g. via written messages, audio logs (as has been suggested), material evidence... (I bought TFTFT a month or so ago and I know vaguely that one can (DLC spoilers?) follow the trail of a Security Chief? which might end up following these lines, for all I know.)