I really like the idea of having a West Marches style public map that shows the known surroundings, to which players can add their own discoveries to share it with other parties. I could simply make a complete hidden map and just uncover all the rivers that a party has explored.
Having players attempt to draw their own maps as they go and the public maps accumulating errors over time sounds really fun to me, but without advanced surveying equipment, what would the characters even be able to scribble down? There's be no way to really track the distance covered over any number of days, and with a winding river it would be really hard to even tell the general direction you're going. Especially through a dense forest.
Any ideas how expanding the existing maps could be made into an activity for players?
One possibility would be to have players who want to map their progress make a navigation check, to determine how accurate their notes of their progress are. WWN makes skill checks with 2d6. You could have the player roll one die and the GM roll the other one, so the player can have an indication of how confident the character is in the accuracy of the notes is.
Then you also could have different maps made by different characters for the same areas, and players might have more trust in the maps of a character known to have a very high Navigation skill level.