Make the iron rarer so that the copper really plays a part of progression.

It bothers me a lot that copper is practically avoidable for any player. When they added copper tools and armor, I thought they would at least reduce iron generation so that copper could function better, but they didn't do that, making copper an extremely optional thing in the initial progression. Leaving the iron rarer in the upper layers where it is more prevalent is extremely important for a smoother progression.