In Pathfinder, there is an item called the Monk’s Robe. I like the idea, and have tried to bring it to 5e, but I don’t know if it is balanced. Thoughts?
Monk’s Robe Wondrous item, rare (requires attunement) 1 lb.
When worn, this simple brown robe confers great ability in unarmed combat. If the wearer has levels in monk, their Unarmored Movement speed bonus and Martial Arts die are treated as a monk of 5 levels higher. If the wearer is not a monk, they gain the Unarmored Movement speed bonus and Martial Arts die of a level 5 monk.
If the wearer is a monk of 16th level or higher, their Unarmored Movement speed bonus is 35 feet and their Martial Arts die is a d12.