Adding new items to the game can have the undesirable side effect of making all items relatively more rare.
This can be prevented by categorizing item drops. For example, any time an item is generated, the game first rolls for category, 10% weapon, 10% armor, 10% food, etc. and only after that, rolls for the specific item. In that case, adding a dozen new types of weapon will raise the rarity of each preexisting weapon, but will have no effect on the relative rarity of armors, food, or anything else.
Is this already how it's done?