How about a monastic tradition based on TMNT?
Ninjas are masters of disguise, subterfuge, and stealth. The operate in broad daylight or under the cover of darkness with the goals of observation, infiltration, and assassination.
Starting when you choose this tradition at 3rd level, you gain the expertise ability as a rogue. You apply the expertise ability to your Dexterity(stealth), Dexterity(acrobatics), and Charisma(deception) checks.
At 6th level, you gain the rogue’s sneak attack ability as a rogue two levels lower than yourself. You may only use this with weapons a monk is proficient with.
Beginning at 11th level, you can move from shadow the shadow undetected or unseen in a crowd, but must choose one. If you are One with the Shadows, if you start your turn hidden and in an area of lowed viability, you are invisible until the start of your next turn allowing you to freely move from dark location to dark location undetected. You are only invisible to creatures you started your turn hidden from. Also, modes of seeing such as blindsight and truesight will still detect you. If you are One with the Crowd, you gain advantage on all Charisma(deception) checks and advantage on all Dexterity(stealth) checks made while among a group of non-combatants to become hidden.
At 17th level, you gain the ability to make more then one sneak attack per round. For each sneak attack you perform in a round, you may pay 4 ki points to perform an additional sneak attack that round.