That is a very fine idea. A weapon consists of a number of elements. A sword e.g. has the blade, a pommel etc. A bow, the wood, the string. Arrows have shafts, feathers and arrowheads. In BG II you could search for parts and have a smith combine them into a cool weapon.
But why stop there? Why not spread ore around the land, the more ore the bigger the weapon, like a Great Sword. A little ore could still make a dagger. A gem can be sued as a pommel gem. A magical gem can be used to enchant the weapon. Shops will carry a range of parts so you can customise a weapon completely.
You can go even further. You can have quests not just to make a weapon but to make one that will kill a certain boss monster someplace. as a sidequest.
