Add teeth and Hatsune Miku
Basically, when you get hit with a melee attack from a boss, you can lose a tooth. This makes no changes to gameplay. When you have collected 10 teeth, you can use those ten teeth and 3 fiber to make a tooth necklace (it would be armor. I don’t know what it would do when equipped though). And you can use it at an altar or something, to summon a merchant item that summons: Hatsune Miku. Now, since some people probably don’t want Hatsune Miku in their world, you would have to enable it when creating your world to remove teeth and Hatsune Miku. The Miku merchant would sell: a new ranged weapon called “Miku Mike” it’s a microphone that shoots out 3 bullets that look like music notes. It starts at level 12. She would also sell armor that looks like the iconic Miku outfit. And finally she sells a music box, which you can place, and set certain notes, which will be played when it’s activating.