If we are getting more, deeper mine levels maybe it's time to add some new rarer metals in to the mining and smithing industry.
Mythical Metals
Orichalcum, adamantium, unobtainium, vibranium, mythril, nightsteel, there are so many to choose from, all with their own lore and interesting fictional properties.
I think it would be cool to have a few mythical metals dispersed at the new, lower cave levels, or even have a chance to appear in tiny trace amounts in meteors. Could make for some new and interesting jewelries and guilds and add some more incentive to dig lower than level 5.
In addition to this it could be fun to also add sources of magma and natural lava at lower levels? Or even tiny vents like geysers that we can harvest from like a local resource node. At the risk of sounding too minecrafty I think it would be very thematic to have a way to find or create obsidian but for making blades and arrowheads.