Thanks for the great thread! 
Personally, I never liked the rules for ‘inferior’ materials. ‘Inferior’ is a relative term; on Athas, these materials are the standards. It’s metal weapons and tools that are superior to them - hence, I believe they should be boosted up, while the ‘inferior’ weapons should be the unmodified rulebook equipment.
I tampered with some house rules for different materials on Athas, actually. Would you guys be interested to see it?
In my games, bone was the staple of the Athas craftsmanship. It was used for everything and anything, all over the Tablelands. The people of Athas have long since cross-bred various animal sub-races for bones with suitable qualities, used for specific tools and products.
It created a whole industry of animal breeders, butchers, bone-workers, craftsmen and artisans who depend on each other for work. Animal breeders raise beasts for specific tools, butchers extract the bones intact, while bone-workers turn raw bone into craftable material. Finally, the craftsmen and artisans make specific items.
Most of bone weapons and tools came from these ‘industrially’ bred races. The specific races are closely guarded trade secrets, as a good bone type can spell a fortune. Of course, bones from certain magical beasts and aberrations would make excellent exotic materials… If you can find a craftsman who knows how to work with them.
I imagine the same goes for chitin and leather. If my players asked me what animal their breastplate came from, I’d reply that the craftsmen would have to kill them if they knew 
Some form of hard bone had crossed my mind, although I hadn’t thought of mekillots specifically. I wonder if declaring it as hard as steel or iron would be credible enough, though…
Absolutely. The biggest problem with bone isn’t hardness, but rather how difficult it’s to work with. Steel can be heated to make it malleable, worked into the right shape, then cooled down. With bone, you can’t really do that… Which means that working with steel-hard bone would be like chiselling steel.
Of course, I imagine the Athasian bone-makers have ways to go around it. They probably use various acids and substances to make bones more malleable, then different ones to harden it once the craftsmen are done. Still, my rule of thumb is that steel-strong bone can only be used for very raw products. Mellikot bone steel-hard prison cage bars: Absolutely. A mellikot bone steel-hard longsword: Not a chance.
I just had a discussion about bronze with @Rhal-othan in this thread To summarise, bronze relies on raw materials that are much more rare than iron. It is a superior metal than iron (not to be mistaken with steel), though, and much easier to work with.
What makes bronze really interesting is that it’s recyclable. You can take a bronze item, smelt it, then cast it into something new. While I think new bronze is not produced on Athas, old bronze items can be used as raw materials… Which can’t be said of iron and steel.
I’m a big fan of desert glass, which is something like obsidian created from meteor impacts. I use it as a substitute of cold iron.
To continue with my bone industry idea: Some animal breeders in my games spike the diet of their beasts with… Specific materials. It’s not healthy for the animals in the least, but produces bones with extra properties. Yes, Athas is hell for domesticated animals.
I imagine so. Glug has its Afghari wood, Urik depends a lot on obsidian weapons (which are another interesting story). In my games, bone craftsmen from different cities all have their trade secrets. Even the nomad tribes probably have some unique kinds of bone, leather, scale or chitin.
While we’re talking about it, I have another question: What do you all think of rules for breaking weapons? I was always a bit on the fence about it. On one hand, breaking your PC’s trademark sword feels really petty; on the other hand, it makes the rules for brawling and improvised weapons more relevant. What do you think?