So I had a thought in one of the various Monk threads. At its core, the rogue already makes a really solid monk, as it cores a ton of the core payload. Rogue's are super fast with cunning action, able to deal extra damage through "weak spots", can become expert grapplers (expertise in athletics), able to soak damage through technique (uncanny dodge). At high levels blindsense and evasion.... all the meat is there. So instead of trying to carve out some special version, why not just do what subclasses were meant to do.... and just make it a rogue subclass.
And so I present a 1st draft.
Rogue Subclass - The Monk
You have honed your skills at a monastery or under a master, learning to master your body and mind to their ultimate height.
Unarmed Technique - At 3rd level you learn to make your body a living weapon. Your AC = 13 + dex when you aren't wearing armor. Your unarmed strike becomes a finesse weapon that deals 1d8 damage. The AC bonus and damage both increase by 1 at 9th, 13, and 17th level.
Slow Fall - Beginning at 3rd level, you can use your reaction when you fall to reduce any falling damage you take by an amount equal to five times your rogue level.
Stunning Strike - At 9th level, when you make a sneak attack with a melee weapon, you can force the creature to make a Constitution saving throw or be stunned until the end of your next turn. You can do this a number of times per day equal to your proficiency modifier.
Purity of Body and Mind - At 13th level, as an action you can end an effect on yourself that is causing you to be poisoned, diseased, charmed, or frightened.
Perfect Self - At 17th level, you gain proficiency in all saving throws, no longer require food or water, and do not suffer the frailties of old age. You still die of old age.
And so I present a 1st draft.
Rogue Subclass - The Monk
You have honed your skills at a monastery or under a master, learning to master your body and mind to their ultimate height.
Unarmed Technique - At 3rd level you learn to make your body a living weapon. Your AC = 13 + dex when you aren't wearing armor. Your unarmed strike becomes a finesse weapon that deals 1d8 damage. The AC bonus and damage both increase by 1 at 9th, 13, and 17th level.
Slow Fall - Beginning at 3rd level, you can use your reaction when you fall to reduce any falling damage you take by an amount equal to five times your rogue level.
Stunning Strike - At 9th level, when you make a sneak attack with a melee weapon, you can force the creature to make a Constitution saving throw or be stunned until the end of your next turn. You can do this a number of times per day equal to your proficiency modifier.
Purity of Body and Mind - At 13th level, as an action you can end an effect on yourself that is causing you to be poisoned, diseased, charmed, or frightened.
Perfect Self - At 17th level, you gain proficiency in all saving throws, no longer require food or water, and do not suffer the frailties of old age. You still die of old age.