To improve trade activities and also force to build "unique" towns it would be great to have resources only found in special places / nodes on the planet.
As an example, atm you can find oil all over the planet and there is no big need of build large tankers (update V1.3 :D) or carry it long ways.
That takes out mostly a large part from the game we see in the real life = transportation of stuff in large amounts.
I can imagine, with a change in that system (why we have so much wood in the desert?) it can give the game much more depth and an additional layer of needed things and could be a good preparation for trains and also more use for boats, maybe an own transportation profession.