I don't think we need yet another skill-we already have quite the bloat of (oftentimes overlapping)skills already. I think that to repair an item, the same labor that is used to create it should be used. So a Weaponsmith would be faster at repairing a sword based o ntheir weaponsmithing skill.
My desire for a separate skill stems primarily from the real-world presence of
actual tinkers, who had the resources to perform routine maintenance & repair, but
not the actual production, of metal goods. If we're going to incorporate more basic realism into the game (a copper pick should be nowhere near as effective as a steel one), certain mechanisms will have to be implemented (softer tools will need repair far more often), and a Tinker labor is the perfect counter to that. In my opinion.
In Europe there's 'eyelet armor'-metal eyelets densely sewn together to form sheets armor. There are surviving examples of armor made from chinese coins stitched to textile backing from both the Pacific Northwest and from Japan, and I have seen an example from Sudan of a breastplate sewn with Iron rings. People experiment a lot.
Turtle shells, Giant Beetle chitin, etc.
Back on the subject of gambesons, the caverns are a vast source of potential weirdness. The trees alone could be quite interesting: Out of the several types, only nether-cap and glumprong have any unique properties, leaving all of the others available. Suppose the wood of fungiwood trees, for instance, was tough, flexible, and
spongy to the feel: Rather weak against sharp blades, but blunt impacts would be largely absorbed and negated, making a fungiwood gambeson excellent for wearing under chainmail--even if it's noticeably stiffer than a cloth/leather one.