Most of the "more interesting" steel and industrial metal alloys require metals that dwarves are very unlikely to be able to produce... like vandium, titanium, aluminum, chromium, manganese, magnesium and the like.
For instance, "stainless" steel is made with chromium in it. Chromium is highly reactive as a free metal, and somewhat difficult to refine.
Here's a valuable resource for chemical compositions of many industrially used alloys.
I'm well aware of the level of metallurgy and chemistry involved, and the required technological sophistication and infrastructure (and even the safety issues, which I'd like to address), and that some of what I want to mod into the game simply wouldn't be possible for a medieval smith, but I quite honestly plan to handwave some of what will be possible as "Dwarfs can do supernatural things with metal".
I don't want to make it easy for the player to do the improbable or impossible, and most things will be done in a way that sticks closely to realism and science-based (atleast logic-based) methodology, but I
will be including metals and ways of manufacturing and alloying them that a 15th century human smith just couldn't produce or replicate, and that other species in the game will not be able to duplicate.
Some of this will be done through alchemical processes (I plan on doing a lot with alchemy), while others will be just be magic, plain and simple.
The main concern is making the game more interesting, more varied, more Fun, in a sensible but not obsessively accurate, way, with the goal here being, not to pile on variety for it's own sake, but to add more situations and choices of how to play/how to run your Fortress, and ultimately, to maximise replayability.
Balanced gameplay will always be a priority. You won't see whole armies of tungsten-clad dwarf legionnaires running around with molybdenum pikes, or anything similar, coming out of even a very mature Fortress, although even the cheapest dwarf metal products will always have a slight quality advantage over most other species in the game.
NW_Kohaku: Actually, I'm just in the process of making a series of mods--4 of them are in the works so far--that I guesstimate will maybe take 20 years to complete.
And I don't want production to become so nightmarishly intricate, or need so much micromanagement, that it takes away from the enjoyment of the game. Building up infrastructure and going off in new technological directions should be something you can choose to explore and focus on, but in the meantime you should still be able to survive and do lots of interesting things, without having to invent a steam powered trip-hammer, or distill sulfuric acid. It's only as you achieve more and more financial success, that you will start to need to put greater thought into defending your Fortress, which may in turn steer you towards more effective measures, but I want most of the new techniques I want to add to the game to be something you can disregard and still have an entertaining experience.
I also don't think I want there to be a time when you feel like you've stepped into the dwarfen industrial revolution, or anything like that, so volumes of production will be carefully restricted.
I want players to be able to do some fairly amazing things, but only a fraction of everything that's possible in a single game, and even then you should only be able to do or create really powerful things in small quantities, for limited times, or atleast very sporadically (like those intermittant waves of early Chinese porcelain production). But, if you make good decisions, and manage to survive and flourish, then when you get your Fortress to some sort of vague pinnacle where you decide you've "won", I want you to have, not only a sense of success and accomplishment, but a feeling that dwarfen creativity and determination can be every bit as powerful and magical as a dragon in flight.