I'd love to see adjustable scarcity levels.
I'd also like to see a greater variety of minerals, in different forms. How about gold crystals? Or maybe some different varieties of emeralds? In real life, gems of a particular hue may only be found in a single small deposit, in the entire world.
Metals, for that matter, can be naturally alloyed, in all sorts of interesting ways.
For that matter, it would be nice to occasionally stumble on an "artifact" level gemstone.
Like something from this list:
http://en.wikipedia.org/wiki/List_of_famous_gemstonesIt would also be nice if there were a broader "tech tree". Steel's ok, but how about putting all that cobalt and nickel I always find to use? Or adding metals like titanium, vanadium, or tungsten to the game? Aluminum's in, afterall. Even anthracite coal could be a nice find.
If there were some different kinds of advanced bronze in the game, and if steel rusted, you might not even want steel, and could work instead on getting your bronze as good as possible. In that case, "Casting" could become an additional metalworking skill (for lead/copper/silver/gold/platinum and their alloys, etc.)--meaning your "tech tree" could branch out, giving you more strategic choices to make.