Two ideas for existing equipment improvement:
Sharpening Stone: A highly prized item that would add dam roll to your weapon permanently. Only one stone per item. Once an weapon is sharpened its ability to be sharpened again is depleted.
Reinforce: A skilled craftsman can reinforce your armour to improve its AC. Once the armour is reinforced it cannot be affected again.

The idea really leaves room for a vast amount of stones and reinforcement types. Lower level stones will be easier to obtain than high level stones. One could even add this as craftsman skill to produce these items.