I think was this offhandedly suggested in the past, but what about giving Kobolds the ability to do something with their namesake ore, Cobalt?
Kobold is a name of German origin. It relates to the fact some of the superstitious miners believed that toxic little goblins were the ones that were responsible for why certain ore killed you when you without any outwards sign smelted it. This is due to a reaction that occurs, since most of the primary Cobalt ores contained Arsenic. When the Metals were smelted, the Arsenic in the Ore would oxidize, and turn into the highly toxic and volatile Arsenic Oxide.
If and when kobolds get back their stealing function, or perhaps get another building like the 'mining tunnel' or whatever it was called that allowed them to create boulders/ores and such, this could be an interesting non-organic alternative to getting a strong syndrome inducing product. You could get arsenic as a possible byproduct of smelting an ore.
Perhaps kobolds even know instinctively how to get the most amount of Arsenic out of this reaction. By smelting away the constituent ore completely, they can guarantee themselves a small portion of Arsenic from the reaction.
Going further into this quick little research, I found that there are also a few historical uses of arsenic being used as a smelting agent itself. It was used in the smelting of 'Arsenical Bronze', which was believed to produce stronger Bronze than just using Tin as an alloy. This is not far off the mark, (if only a little). Adding Arsenic to the process would do three things
1: It would act as a de-oxidizer, preventing bubbles/pores from forming in the Casting process.
2: It would impart a better hardening to the metal, compared to using pure copper in the process,, it would see a 10-30% increase in hardness and Tensile Strength
3: In the correct percentages, it can contribute to a silvery sheen on the metal, if the top layer of metal was arsenic rich.
Continuing with this ramble, it might be an interesting later game addition of Kobolds could possibley create Arsenic Bronze, or maybe more appropriately call it 'Kobold Bronze'. It would not stand up to stuff like other races late game materials, but perhaps thanks to a little kobold magic, the metal itself could induce a syndrome. You're fine as lond as your armored up enough against the kobold, but if there is one chink in your armor, the little kobold weapons will get ya.
Course, these sorta reactions would not be without their own shortcomings. I can see both the smelting process, and the Casting process carrying the risk of killing the kobolds involved. So you could have this neat Toxin, and neat weapons, though they will be crappy quality unless you wanna risk your best workers in the process.
