While struggling with my “waterbender” monk (I’m still not really happy with it. Whatever.) I spent a lot of time looking at the monk chassis. Here are some of those thoughts.
I really, really like the monk as a general purpose mystic warrior. In particular the shadow monk plays to this role really well. The monk’s assumed-disciplined background plays well against the paladin, barbarian, and rogue; they represent a more spiritual nature than the pally, a more logical one than the barb, and a more highminded one than the rogue. My time playing Assassin’s Creed has convinced me that the main characters there are, in fact, monks.
But D&D stuck the assassin under the rogue (… and the ninja under the way-of-shadow monk). The really important detail for me is the assassin’s one-hit-kill, a death strike which the existing Open Hand monk retains as a capstone ability. If I want something in between, I’m rather out of luck.
The biggest thing missing is a focus on a leading attack. Rogues tend towards single large hits: while they might want multiple attacks, they only get to sneak attack once per turn, so they dual wield to increase their chances of landing the single blow that counts. That’s perfect for the assassin rogue, who drills down to a single large hit as the first move in the combat, maximizing their damage against a foe which fails a perception check (surprise) and a dexterity check (initiative) both. There’s no real equivalent for the assassin monk. Until now.
So: Way of the Assassin.
At 3rd level, you have advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet, and any hit you score against a creature that is surprised is a critical hit. You also gain proficiency with “assassin weapons” (blowgun, if you’ve got rules for ’em, garotte), and the poisoner’s kit.
At 6th level, you can use a bonus action on your turn to give yourself a rogue’s sneak attack progression equal to your monk level until the end of your turn. This ability still requires a finesse weapon (not unarmed, quarterstaff, spear, etc; daggers, short swords, daggers etc fine) and the conditions which trigger a sneak attack are still required (advantage or an adjacent ally). Reasoning: I want to give you sneak attack like a rogue, but you have Flurry of Blows and Martial Arts to contend with. I feel no shame about Flurry of Blows — it costs a ki point, after all — so let’s consider Martial Arts. That bonus action is, say, 75% likely to deal one more hit’s worth of damage — a roguelike sneak attack progression, starting at 3d6, seems about right. Over time, it gets better and better, until it reduces Flurry to a mook takedown maneuver, or until you’re not able to get in position to trigger sneak attacks. Since you lack the rogue’s cunning action, that seems pretty common; since you lack a bonus action, you won’t be dual wielding your sneak attack mechanism. Hurrah!
At 11th level, you gain the Way of Shadow monk’s Cloak of Shadows. In addition, your unarmed strikes gain the “finesse” property. Reasoning: Hello, unarmed sneak attacks.
At 17th level, you gain the Open Hand monk’s Quivering Palm.
Back to more general notes: The monk uses wisdom (because of the monastic –> cleric) connection. Wisdom is a poorly defined ability. Given my druthers, I’d probably switch all spellcasters away from it — clerics cast off of charisma, druids off of intelligence — and switch the monk to intelligence too. And switch warlocks over to intelligence based casting while I’m at it. And give the cleric and the warlock (and the paladin) the same “schema” for casting; both use renewable slots or neither. The damage this does to the healing system is far secondary to the relief it offers to my OCD that things-that-work-similarly-in-fiction-work-similarly-in-rules-structure. Making druids mostly known-spells (instead of prepared-spells) would help too, guys.
Er, anyway. Making monks intelligence based goes some ways towards making them easier to use in non-wuxia-like settings. High elves get a dexterity and intelligence boost, and should be monks with their focus on refinement and study. Their focus on concrete techniques shows a need for self-understanding, but not mystical communion with an external force, save perhaps Ki — there’s no reason Ki and (the Forgotten Realms’) the Weave aren’t very nearly the same thing. Intelligence-based monks would have reasons to have high stats in calligraphy, languages, history; all those things we want our monks to have. Bruisery-monks (more physical than mental) are all well and good, but they should be motivated by giving monks things separate from their casting stat; they already don’t work because they should be Dex-based, not Str-based!
Oh, while we’re here, assassin weapons. I think we’re good with our poisoned daggers and blowgun needles. I think we need to add Garotte, a special martial weapon. You can only attack a target you have grappled with a garotte. The target becomes restrained while grappled and cannot speak or breathe while restrained in this way and the garotte gains the finesse quality, dealing 1d6 damage, against the restrained target.
A pad soaked in ether: Similar to the garotte, this weapon can only be used against a grappled target. The first time each round it’s used as an attack, the target is exposed to the (presumably inhalant) poison in which it’s soaked, and cannot speak or breathe until the start of your next turn. That’s all.