Metals. For a high-tech race it's weird that their best metal is mithril, imported from elf.
I don't want them to wind up with too many advances in the area of making super-strong metals - dwarves should still be the best metalworkers. Not all technology has to advance at the same rate as it did in real life.
I do think that it makes sense for gnomes to have better access to aluminum than other races, though. In real life, the process that extracts aluminum from bauxite requires electricity. That's why aluminum used to be so valuable, but is now commonplace.
The old Masterwork made aluminum extraction from bauxite available to all races, although this isn't realistic given the technology levels (unless it's explained with magic). Whether Meph wants to keep it in or allow it to be exclusive to gnomes is up to him. To be fair, aluminum is kind of useless for weapons and armor, it's only useful in DF because of its value and rarity... which is sort of ruined by being able to extract it from bauxite.
It
is useful for robots, though (in real life - right now there's no script in place for making lightweight robots faster, although that would be a sensible upgrade to the construct-creature script).
Maybe instead of making stronger metals, gnomes could make a lightweight, reasonably strong aluminum-steel alloy. Although automatic machines making alloys is still a problem I haven't fully figured out (because as they work now, the machines process items one at a time).