It could be very entertaining to add credo "Metal Alloys". Just a concept:
# level 1 - Ability to find 3-4 new ores. Those ores could be rare to very rare; mine-level dependent.
# Levels 2-3 - To open few new metals that could be obtained only in pebbles (like gold now) after smelting from those new rare ores
# Level 4 - Forging benefits
# Level 5 - The ability to make alloys by mixing different metals on the basis of steel. new metals could serve as crucial additive pretty much like spices now in food. Some alloys could possess higher strength, for example leading to better weapons.
A CATCH: the exact mixing formula is unknown and has to be learned by trial and error. Different World could have few secret recipes that lead to superior metal abilities, while most other recipes would not change it or even might even degrade it.