Personally I'd love to be able to create weapons and armours from scratch; mining for the raw materials and going through the neccessary procedure to craft the items. It gives a more personal feel to the weapon or armour and with things like customizable design templates or customizable attributes based upon how much of a material you have or your skill it could really be great.
Gathering different materials could also be good not just for armour but for clothing. You could aquire certain pelts and fabrics and create a fur hooded cloak or you could kill a cow and tan the hides in order to make leather to create leather clothing items and armour. Again, depending on how much of the material you have and how high your crafting skill is you could make a variety of items.
Another thing I'd like to see is being able to wear certain types of clothing underneath armour, because a metal armour against bare skin is a little silly...
I really think this would work well if it was implemented properly. What do you guys think?