Invisible Ink

Invisible Ink could be placed on signs and armor and would make the text on the sign or the armor invisible unless viewed with a spyglass, This would give the spyglass more real uses and would give people a way to make their skin visible instead of their armor if they don't like the size it adds to their player.

My suggestion for a crafting recipe is to make it so you have to brew glow ink with a golden carrot, then a fermented spider eye just like a potion of invisibility.