In MoM there are rare resource nodes called mithril and adamantium.City that controlls one of those can build troops with special weapons and armor. Controlling such resources and building such troops is one of the coolest features in Mom for me.

In FE there is iron for mundane weapons and crystal for magic stuff and it works pretty well. But another icing on the cake for me would be to to include the above resources.

How would could it work in FE?

My idea is for them to work like an overlay over ordinary(mundane and magical) weapons/armors. Each weapon/armor would have a mithril and adamantium variant. Visually each weapon/armor would use the same 3d model but have a different texture(glowy silver for mithril, glowy gold for adamantium).

Mechanical effects would apply like a trait over existing weapon/armor designs and could work something like this:

Mithril effect(magical and mundane stuff): Weapons  and armor have half normal weight,  weapons(including staves) get +10 accuracy and + 1 initiative

Adamantium effect: Weapons(including staves) do 50% more damage, armor blocks 50% more damage

How could you plug something like this in the current UI?

I think the simplest way would be to add a check-box in the unit designer that activates the variant version of the equipped weapon/armor. That option should be active only if a player controls corresponding resource node.

How would it affect game balance?

I think that the main focus should stay on iron and crystal,  so they should be very rare resource nodes, maximum 1 per map. They would generate resource very slowly, so at most a player could have a few regular units equipped with adamantium/mithril stuff but they would be quite special and fun