Give your shopkeeper the ability to enchant tools.
This could be sold at the team upgrades block, and would make the “enchant” section of the shop appear. After paying some emeralds, you will then have an enchanted tool!
Swords
You would be able to put Sharpness and Fire Aspect on these, after paying 5 emeralds for level I, and 7 each subsequent upgrade.
Bows
After swords, comes bows! You could put Power or Punch on your bow at a base price of 3 Emeralds, or 5 for each subsequent one.
Pickaxes
For 12 Diamonds, you could put Efficiency on your pickaxe.
Axes
For 12 Diamonds, you could put Sharpness or Efficiency on your axe.
(Sharpness idea by @Stellar6666)
Armour
You could put Protection, or Projectile Protection on your armour for 10 Emeralds and 5 Diamonds, with it becoming 10 Emeralds and 10 Diamonds on subsequent purchases.
(The shop would be a separate tab in the main shop)
(All prices are merely suggestions)
also good idea, voted