I'd love to see a 100% completion mechanic for this game, maybe with a unique hat reward per world.
It seems like the logic is kind of already present, since the game knows when you've first discovered something. Using that same logic, I'd like to see a tab on the character sheet named "journal" or something that lists off all the things I've discovered, with completion statistics for each topic. Boards chopped, blocks sawed, bugs found, creatures butchered, etc.
It wouldn't show you anything you haven't discovered, but it would keep track of what you have discovered and give you a 82/196 read out telling you how many more things in the category you have left to find.
I think it would add an interesting bit of playability to the game, and give collection-oriented players something to strive for on each world. I also think it would be neat to walk around with the completion trophy on my head, something that tells all the players who see me "Look what I did!"