I did some searching around and really only saw stuff relating to the existing minerals.
On a good embark location, the weapon/armor tiers go copper < iron < bronze (it's slightly better through some SCIENCE research as far as I can tell) < steel, then a giant jump to adamantite. Whenever I play, I find myself just pumping out bronze, then steel, and last adamantite.
I was thinking and maybe some new fantasy materials would be interesting. Like deeper down, finding mithril, a weaker form of adamantite perhaps. Or some of those other ones you see in fantasy and games. Something to fill the gaps and add some diversity, like a heavy material that has armor effective against piercing and is heavy for blunt weapons past steel.
This would probably have some problems with being a bit gamey though since you wouldn't want it on the surface. Like only finding it on the rock levels where the second or third cave levels are. This could tie into the idea of other planes though. On the "normal" plane, you see the normal set up, but perhaps on the elemental plane of earth, you could find normal the normal metals and mithril or orchilium deposits.
Sorry if this is all over, new to the forums.
Edit: You see this type of thing in overhaul mods like masterwork and genesis, so hopefully people understand what I mean.