Had some ideas about metals and metal alloys.
Antimony : Somewhat known by alchemists since antiquity. It is a hard, brittle metal on its own, but makes some interesting alloys.
Bullettine/Bullet Metal : Lead + Antimony : An alloy of lead and antimony gives most of the density of lead, but much harder and more useful. I have heard 2 to 5 times harder than pure lead, depending on the mixture. In our world, this alloy is used for making bullets and "type metal", for printing presses.
Modern Pewter : Tin + Copper + Antimony : A modern type of pewter without lead in it.
Antimonial Bronze/Antimonze : Copper + Antimony : I imagine this would have properties similar to Bronze, as it's Copper + Hardness but I'm not sure. Can't find data. Possibly similar to Arsenical Bronze though.
Paktong/Cupronickel : Copper + Nickel : Chinese for "white copper" known to them as a rare form of copper, it turned out to be copper and nickel. I've used this in modded games with data from 50/50 cupronickel, and it's comparable to bronze and iron, maybe a little closer to iron. Very useful, but hard to find, though perhaps no harder than Tin Bronze.
White Gold : Gold + Nickel : A modern alloy, but since dwarves have nickel, they could make it.
Nickel Steel : Steel + Nickel : Supposedly a better form of steel, but I can't find data on it. It can also be decorative.
Irogane : A group of "colored metals" produced by applying rokusho to various alloys. No idea if this falls exactly into DF's tech cutoff. Supposedly it's "at least 600 years old".
Rokusho : Recipe:
https://en.wikipedia.org/wiki/Rokush%C5%8DShakudo : Gold + Copper + Rokusho : Black in color
Shibuichi : Silver + Copper + Rokusho : Dark Grey or Blueish in color
Akagane : Copper + Rokusho : Red or Brown in color
Mokumegane : "Wood grain metal", multiple kinds of metal in a stripey pattern that resembles wood grain.
There are others, but these seem like plenty.
Tumbaga : A gold + copper alloy used in mesoamerica. Acid is used on the surface to give it a more golden look than rose gold has.
Panchaloha : Gold + Silver + Copper + Zinc + Iron : A holy alloy from India.
Ashtadhatu : Gold + Silver + Copper + Zinc + Iron + Tin + Lead + Antimony : An even holier alloy from India (I assume).
Sky Iron : Iron + Nickel, but ideally a natural ore : Meteoric Iron, also considered holy by some people.
I've also been thinking about iron and steel. Iron requires a more complicated process to smelt, and there are different processes of arriving at iron and steel which lead to different types of products.
Bloomery : Basic Iron Furnace
-> Wrought Iron : Iron Ore
-> Pattern Weld : Wrought Iron + (Wrought Iron + Carbon)
Crucible : (unknown in most of Europe, but existed in India, Central Asia and possibly Scandinavia, made by Ulfberht) (I'm not sure how the building requirements would be different from a Bloomery)
-> Crucible Steel/Wootz/Ulfberhtine : Wrought Iron + Clay + Clear Glass (the flux) + Carbon
Blast Furnace : Unknown in Europe until Industrial Times, but China produced them in antiquity, forgoing Bloomeries entirely. Diagrams from the 1300s show a blast furnace powered by a waterwheel, so I envision this DF workshop would require power to work.
-> Cast Iron : Iron Ore (More brittle than Wrought Iron)
-> Blast Steel : Cast Iron + Time (maybe + Stirring Rod?) (I'm a little unclear on the exact process, I think as long as you have the blast furnace, you just have to wait for the cast iron to decarburize)
-> Allegedly, one could use molds to cast steel with this type of furnace, but I'm not sure
(Clay walls act as flux?)
Tatara : The Japanese technique
-> Tamahagane : Iron Ore + Iron Sand + Carbon (useless lump, used for folding process)
-> Laminated Steel : Tamahagane + Folding (there are more complicated ways that this reaction could be accomplished)
(Supposedly, the laminated steel is more or less the same process as the pattern welded blade, and could maybe be a further step after pattern welding. (so Wrought Iron -> Pattern Weld -> Laminated Steel) Supposedly, Blast Steel is superior to Laminated Steel. )
*To my knowledge* the current DF method of steelmaking is apocryphal. Using Flux Stone is the modern way we make steel, not the medieval way. However, it is mostly a fantasy game, it might be interesting to be able to use flux stones in a blast furnace to produce modern "monosteel". I'm not sure what else would be required.
Magma Well : The use of Carbon in all the real-world furnaces changes the chemical composition of the iron, producing steel. (I'm actually not sure how a Magma Forge is supposed to work, since magma isn't hot enough to melt iron) Using a DF Magma Forge would change that, because there is no burning carbon. But magma is made of useful minerals itself, such as Iron and Magnesium. Given that this is all fantasy, you could potentially have a way of extracting Iron and Magnesium or an FeMg alloy from magma itself. And of course, one would expect the ability to extract simple obsidian.
Nickel might also need to be extracted from iron furnaces, since its melting point is also quite high.
And some fantasy "metals":
Ironwood : an underground tree that is as strong as iron.
Dracometal : from Dragon Fossil ore, the bones and scales of ancient creatures, they can be melted down and forged like metal.
Pyrochalcum : from Pyrolith ore, it is extremely sharp
Geochalcum : from Geolith ore, it is extremely dense
Aerochalcum : from Aerolith ore, it is extremely lightweight
Hydrochalcum : from Hydrolith ore, it is extremely beautiful/valuable, and can be woven into thread.
Neutronium : an incredibly super dense material. A marble-sized amount weighs as much as a mountain. It is unstable, and combining it with the right materials will cause it to rapidly expand into still very heavy but useful materials that can be forged like metals.
(I mod these into my games to appear as small clusters, like gems)