I'd like to propose that a wide variety of secrets, hidden objects, hidden mysteries, and these sort of things be given a thought as to be added. I believe it would flesh out the game world, and give it plenty of life and intrigue.
The idea was born during a recent playthrough, trying to survive in the most remote northeastern part of the map. Would it not be intriguing for some of these places to hold mystical or perhaps strange qualities/events?
Just a few suggestions here on things that could be considered "interesting secrets", this is absolutely not an exhaustive list:
The great northern waters somehow giving a glimpse into the afterlife/ancestors (I believe the game makes reference to something like this in the in game library? Or perhaps I'm just remembering it from Kalevala)
Ghosts or frightening apparitions
Remains of earlier cultures, perhaps ancient burial sites or something of the sort
Abandoned/destroyed houses or villages
Or any other interesting little thing that is not advertised to the player, only discovered accidentally or by very little chance indeed. Of course, if these things are added, one need not say which things are (or even that they are at all) in any patchnotes, leaving it for the player to discover
Just a thought