Could be small things like gems in mountains or underground, or beautiful flowers that gives a housing bonus. Or truffles that are used in highend food but fragile to climate change and will disappear or spoil as one of the first consequences of pollution.
As long as these resources are scarce, they will be fun to go hunt for in the world. And it could give another incentive to keep nature non-polluted.
Right now, scarcity in the game only comes from high demand, but nothing in the game is intrinsically scarce and worth looking for, which is a shame. Exploration really isnt a thing, which it could be.