I was pondering whether some of the unusual folk might be vulnerable to -- oh, platinum. Or aluminum. Metals not known until science devised new ways to find them. An unpleasant surprise perhaps.
On the other hand, I don't know if any creatures are vulnerable to some anciently known metals. Tin. Gold, even. Lead -- I have heard of cases where lead from church windows is dangerous, but not lead in itself.
And then there's the question of whether they are random and wild enough for a proper vulnerability compared to some of the wacky stuff you find in folklore. A catalog of metals is a bit standardized when compared to such weaknesses as a pentacle, soybeans, or four-leafed clovers.