In case you didn't already know, I'm designing an RPG.
One of the things that came up recently is crafting (and associated abilities) and I had to admit I hadn't put a lot of thought into it yet. The design I have so far goes along these lines:
Techniques, ingredients, and equipment all factor into your final product.
Each contributes something to the end result, and there will almost always be a trade-off.
You include Tempering when making your sword, it makes your sword more durable but it adds an extra three hours (gametime) to the production time.
You also have an extra hot forge, which lets you work faster but requires more fuel.
The higher your skill, the larger the bonuses and the smaller the penalties, and you can also combine things to help cancel out negative effects.
Some of the effects will be related to the process (how long does it take?), and some will be permanent aspects of the result (weapon durability).
so perhaps you want to tweak your weapons for high damage, or lightweight, or mass produce them really quickly.
there will also be somewhat seperate metallurgy skills, for alloys and smelting, so smiths will have a variety of metal sources to choose from.
that means you have the design for a sword, and you have the choice of making it out of iron, steel, bronze, silver, or Alloy X.
Alloy X was made by a metallurgist to have what he deemed the best combination of weight and strength, and then sold to the smithing community.
I think the flexibility and constant discovery will make this an enjoyable system. And of course, we'll be providing wiki space for the players to keep track of their findings, and also adding new elements to the mix occasionally to keep things interesting.
Please post if you have any thoughts or think I left something important out.
