I would love to see flintknapping added into the game. Smash 2 stones together, get a chipped stone out of them. It's obviously a little more complicated than that, but it's an incredibly primitive skill that people would have no problem picking up. It would simply be a crafting recipe, possibly with a foraging level as a prerequisite. Stones are mostly useless and very plentiful while chipped stones are rarer and actually useful so I think this would be a nice addition on that basis.