Here are a couple of suggestions about the possibility of certain "advanced" weapon types only being forgeable by smiths of correspondingly high skill.
To give you some suggestions: Any weaponsmith of any skill level could forge a basic knife, but in order to forge a switchblade, it makes sense that you'd need knowledge, or insight, into how a switchblade is put together, and thus more skill. You need more skill to create a Danish longaxe than you do a hatchet, and two-handed flamberges are just naturally more difficult to make than shortswords.
This could be applied to a very wide variety of weaponry in the game--where you'd start by having your soldiers outfitted with hatchets, slings, etc. but these would evolve over time into bec de corbins, rapiers, partisans, double-crossbows, etc., as they could be expected to evolve in real life.
Please note that this isn't a suggestion for "new technology", ala Civilizations etc., but rather an entirely skill-based exclusion system, so that as soon as you had a legendary weaponsmith, you could immediately make any weapon that was available.
Hopefully, this would make for both a more realistic forging system, while at the same time allowing for more exotic/interesting/powerful weapons in the game, without unbalancing the game at the same time.
It might also be a good excuse to greatly extend the timescale required to go from dabbling to legendary, since it would be a lot less fun to play around with such a system, if in two years you automatically got intimate knowledge of every weapon in the game, no matter how exotic and rare.
I'm not sure what Toady has in store for us, as far as "knowledge", but this seems like it might be a good case for having "mental" vs "physical" skills in the game, in that you might have a Legendary weaponsmith, with the knowledge of the design of literally hundreds of different weapons, but who has only a dabbling skill at the actual process of smithing, giving you the ability to equip your soldiers with a bunch of really cheap, poorly made meteor hammers and seige arbelasts. On the flipside of that, you might decide to set out with someone who really knows how to forge things, but who's never been out of the smithy, allowing you access to *iron club* after *iron club*. So it'd be a tradeoff.
Once your Weaponsmith (maybe change the name to Weaponmaster? and use the Weaponsmith skill to determine quality?) was able to make a weapon you particularly liked (spontoon, for example), you wouldn't be forced to wait until he/she knew how to make a Bohemian earspoon, you could just switch them to learning how to actually forge spontoons.
An alternate suggestion for this is to come up with a set of basic weapons (knife, hammer, club, pick, hatchet, spear, sling, what have you), and give each of them it's own subskill. As any given dwarf creates one of those items, they would advance in that weapon's "skill", and at some point, the skills would branch off: knowledge of knives turns into knowledge of daggers, hammers into maces, clubs into shortswords, etc., each again, with their own subskill. Allowing you to guide the "learning process" into the kinds of weapons you want, in a more natural way--giving you the opportunity to either specialize (getting the precise weapon you want, as soon as possible, and then switching to skill in forging very early, which translates to higher quality), or allowing your craftsdwarf to become a generalist, which would give you advantages in both trade (giving the customer what they want, and giving your nobles what they demand, more often), and allowing you to better adapt to circumstances (where's that brandistock when you really need one?)
Again, it's not modeling technological advancement, it's just an individual craftsdwarf learning more about the trade, and becoming more confident about what he/she can achieve, in a way that I think would be more realistic, fluid, and fun.