Could the animal training civilization experience system be expanded to metallurgy? Maybe we need to learn how to make steel and maybe someday we can learn to smelt cobalt. And then maybe cobalt steel which is better than steel.
There are several suggestion threads that cover the nature of learning or "tech trees", such as this one, which is active now: http://www.bay12forums.com/smf/index.php?topic=109129.0
The thing is, tech tree ideas generally aren't very popular among DF players, although metallurgy is an area where there might be more room than others.
I imagine the biggest trip up with tech trees is that people don't want to have to grind a skill to obtain a level in competence, but I like the idea in someways due to the fact it can be done a little differently than normal.
Like in animal training, eventually training a Black Bear should improve a general Bear training skill which in turn allows you to train other bears, like say Grizzly Bear more easily allowing you to gain skill in the specific species easier. Though this would require defining creatures to a Genus-level (not necessarily a literal scientifically defined Genus, ie Owlbears or Panda Bears can be considered Bears for the purpose of being a Bear-like creature). On that note, it could work like spheres (but spheres players can define if they create a whole new class of creature) and stack several onto a creature to show how it relates and how experience in training grows. For example, specializing in creatures that are Bears are possible, but bears are also Carnivorous Mammals. Knowing how to deal with one type of carnivorous mammals should be able to supply some experience with others, but not necessarily a lot of experience. So in my idea, the skill tree works backwards. You train a specific animal, but gain experience in related less specific fields which in turn allows for easier training of other creatures later on.
Granted, this is so complicated I wouldn't expect Toady to pull this kind of overhaul into a system he just added for a while(read: years). But it is food for thought, y'know. This system allows for a lot of interesting potential in how you approach your world if it is further expanded. I said before, this system could be used to give different civs of the same species a unique culture. Even now you can already see a bit of this. My current civ lives on the border of a glacier biome and have adapted to being able to training some of the creatures found there. The other civ I did a check embark on was on the border of a jungle and had a good deal of lion and tiger training going, iirc. (That said, is there a way to check a civs training levels without embarking?)
I'm starting to train some Grizzlies recently, I caught some war Grizzlies when the elves attacked and one had babies. You know the cubs were immediately domesticated! They received their mother's training from the elves, so all I had to do was break the babies born in my fort and they are nice little puppies! Now I can turn all the elves' hard work against them in a single generation! MUWHAHAHAHA! Edit: In fact, I'm planning on making a War Grizzly Production Plant with the War Grizzlies I caught. The cubs are a cinch to train, then I wait until they are old enough for WAR!