Changes
Island -> replace Castle Vantress
? -> replace Sensei's Divining Top

Need:
$$ Throat Slitter - Removal on a ninja
$$$ Deadly Rollick - Free removal that counts as cmc 4
$$$$$ Fierce Guardianship - Free counterspell that still does some damage
$$$$$ Vampiric Tutor - Get a high CMC card to top in response to Yuriko trigger, get an extra turn by tutoring Temporal Mastery just before my turn, or just get what I need in an emergency
$ Conspiracy - More ninjas
$ Xenograft - Cheaper alternative to Conspiracy
$$ Unnatural Selection - Turn things into ninjas at instant speed
Thalakos Seer - "Unblockable" that draws a card when ninjutsued out
$$$$ Volrath's Stronghold - Get back a creature and secure a Yuriko trigger

Consider (have):
* Phantasmal Image
Null Rod - I don't have a lot of artifacts that I will lose to this
Universal Automaton - Vanilla ninja
Phyrexian Walker - 0-mana dork
Street Wraith - Cantrip that hurts a lot for my opponents
Chain of Vapor - Cheap interaction
Gitaxian Probe - Get some info and cantrip
* Scroll Rack
* Dispel, Flusterstorm - Cheap interaction
* Ashiok, Dream Render - Clear the top of my deck, hate on graveyards and prevent all kinds of searching including tutors and land-based ramp

* Mystical Tutor - Steal from Rashmi?
Tormented Soul - Another 1 mana unblockable
Krydle of Baldur's Gate - Can scry on connect, and can give unblockable to itself or something else
Looter il-Kor - "Unblockable", cheap looter
Riverwise Augur - Brainstorm on a bounceable creature (can get unblockable with Tetsuko)
Rapid Hybridization, Pongify - Cheap removal
Arcane Denial - Cheap counterspell
Price of Fame - Removal that costs less than it seems (if I want to kill a commander or other legend), and gives some topdeck manipulation in the same go
Sage Owl - Cheap flier that looks deep
Thassa, God of the Sea - Smuggle as many things as I want for cheap, on a hard to remove permanent with scry as an extra upside
Cyclonic Rift - Goes in all blue decks, but maybe this one can do without?
Mystic Confluence - Counterspell that can let me re-use something
Spark Double - Copy Yuriko! (or Etrata, I guess)
Whelming Wave - Cheap board reset, but it doesn't deal with the occasional leviathan etc
Fact or Fiction - Card draw that can help fill up the graveyard for delve
Ravenous Chupacabra - Reusable removal
Decree of Pain - Boardwipe for small dudes that hits above its real cost with Yuriko, or very expensive board wipe for everything that draws me a bunch
Disappearing Act - Familiar's Ruse for 1 more but can bounce non-creature permanents
Kindred Discovery - Basically draw two cards per ninjutsu
Thalakos Mistfolk - Basically unblockable and can save itself (or guarantee a drain with Yuriko) by putting itself on top
Ancient Excavation
Flitterstep Eidolon - Cheap, unblockable creature that can make something else unblockable in the late game, but it is very expensive to do so
Abhorrent Overlord - Great card to replay after being bounced with ninjutsu, and creates extra bodies to bounce, but I'm probably not going to have a lot of devotion if I'm bouncing my stuff every turn
Cryptic Serpent - No evasion or abilities, but it has a high CMC that can be heavily discounted if I play enough spells
Champion of Stray Souls - Can put itself on top of the deck, and can sacrifice small creatures to get back better ones
Aetherling - Unblockable, resilient win con that can trigger LTB-abilities
Canal Courier - Can be an unblockable ninjutsu target, and becoming the monarch every time someone steals the crown is nice
Keymaster Rogue / Storm Sculptor - Can trade places with a ninja every turn, but can't be played initially if I don't have any creatures out
Naga Oracle - Topdeck manipulation that can also fuel delve while being reused with ninjutsu, but quite expensive for the relatively minor effect
Gravepurge, Forever Young - Get back a bunch of creatures or put a big one on top for big Yuriko damage
Clever Impersonator - Ninjutsu is a good way to reset clones, and this is a good one to reset
Balustrade Spy - Rogue with some evasion and an ETB that can kill someone if re-used enough times
Amphin Pathmage - Can smuggle things through with spare mana, or be used for ninjutsu itself
Agent of Erebos - Reusable graveyard hate
Academy Elite - Slow card draw that can be reset with ninjutsu
Offalsnout - Graveyard hate that can be reused with ninjutsu, but kind of slow. Can be used once with instant speed, though, or cheated with Crystal Shard etc
Jeskai Infiltrator - Can be unblockable, and works nicely with topdeck manipulation
Crafty Pathmage - Can smuggle small dudes in for free
Nether Traitor - "Unockable" and can come back if my ninjas die
Aether Adept - Bounce back a ninja or something of my opponents'
Minamo Sight bender - Can smuggle anything, but it's going to be expensive for bigger things
Snapcaster Mage - Great thing to repeatedly bounce
Lochmere Serpent - This is very big and I probably won't want to bounce it, plus sacrificing an island is a high cost to gain unblockable if I'm just gonna ninjutsu it
Slip Through Space - Gives unblockable and replaces itself, but probably not good enough to be worth a spot
Glaring Spotlight - Can smuggle a whole team once, and up until then it turns off my opponents' hexproof
Skeleton Key - Makes dudes evasive and combat looters, but skulk is probably not good enough to reliably smuggle dudes
Spreading Plague - Continually wipe away all U or B creatures if I'm fine with only having one myself
Perplex - Counterspell that can be used to get a 3-mana unblockable dude
Artful Dodge - Smuggle twice, but is that enough?
In Garruk's Wake, Plague Wind - Strong boardwipes that hit for a ton with Yuriko trigger, but getting that much mana is probably going to be hard
Prowler's Helm, Hot Soup - Make any creature unblockable, but the equip cost is a bit steep if I'm just going to ninjutsu the creature anyway
Genesis Chamber - Get dudes to ninjutsu out
Mask of Riddles - Evasion and card draw, but the evasion is limited and if I'm going to ninjutsu the creature anyway, the cost might be a bit steep
Hero's Blade - Deal more commander damage or pump other legendary ninjas, but it doesn't help me if I'm not already attacking
Field of Reality - Reusable unblockable aura
Thopter Spy Network - A source of evasive disposable dudes to ninjutsu with the possibility of extra card draw
Pondering Mage - A Ponder on a body that can be ninjutsued, but probably a bit too expensive considering no evasion
Peel from Reality - Save or reuse one of my dudes while also bouncing one of my opponents' guys
The Magic Mirror - Hits like a truck off of Yuriko, but with the current build it's not gonna be cheap
Ghastlord of Fugue - Unblockable and has an effect when it connects, but the effect is kind of minor
Shadowmage Infiltrator - The evasion is not always reliable, and he is somewhat expensive with that in mind. Does draw cards if he connects, though
Tomebound Lich - ETB trigger, connect trigger and people have incentive to not block it, but does it fit? The looting fuels delve
Flood of Tears - Reset the board and possibly put a dude back ready for ninjutsu

Lands:
* Underground River - Untapped fixing
* Flooded Strand - Untapped fixing and a way to reset the top of the deck
Faerie Conclave - Evasive manland
Fetid Pools - Dual land with basic types, that can cycle when flooded
Witch's Cottage - Can put a creature back on top and come in untapped, but 3 other swamps might be hard

Consider (don't have):
$$ Curtains' Call - 3 mana to kill two creatures, while the CMC is double the real cost
Baleful Mastery - Removal spell that can be cast for less than its CMC
Hagra Mauling - Another land that hurts
$ Teferi's Veil - Protect my board from sweepers between turns
Spite // Malice - Split card counterspell or removal
Deprive - Counterspell that lets me re-use lands
Keeper of Keys
$$ Equilibrium
Telling Time - Cantrip that also sets up the top of the deck after
Ethereal Usher - Give things unblockable or tutor for Ink-Eyes etc
Hostage Taker - Removal and value that can be reused with ninjutsu
Deepchannel Mentor - Make half my stuff unblockable
Aven Fateshaper - Repeated topdeck manipulation, but very expensive
Architects of Will - Reusable topdeck manipulation that can be cycled when it's no longer needed
Stronghold Rats - Basically unblockable, with some decent value if it connects
Slippery Scoundrel - Unblockable and untargetable, but only late game
Neurok Invisimancer - Unblockable and can smuggle through another dude each time it enters
Cephalid Pathmage - Unblockable dude that can smuggle another creature by sacrificing itself if necessary
Big Game Hunter - Kills something big on ETB
Watcher for Tomorrow - Nice ETB/LTB abilities
Passwall Adept - Cheap creature that can smuggle in others
Wharf Infiltrator - Evasive looter with payoff
Skeletal Changeling - Ninja that can regenerate, but no other abilities
Fogwalker - Evasive, cheap dude with an etb
Departed Deckhand - Evasive Smuggler, but fragile
Daring Saboteur - Looter that can be unblockable, but has to pay for it
Coralhelm Guide - Can smuggle anything, but for a high cost
Dauthi Jackal - "Unblockable" dude with removal on it
Dauthi Trapper - Smuggler
Dauthi Mercenary - "Unblockable" dude that grows
Blighted Agent - Unblockable and infect
Amoeboid Changeling - Ninja that can turn anything into a ninja
Aether Swooper - Evasive dude that can make more evasive dudes
Shadow Rift - Smuggle something for cheap without costing a card
Footbottom Feast - Stack the top of the deck with any number of creatures from the graveyard to get back important stuff or just to guarantee hits from Yuriko triggers
Traitor's Clutch - Can smuggle something twice, or be looted away and cheaply used from the graveyard. High CMC on the front end for hefty Yuriko triggers
Diabolic Vision - Rearrange top 5 and draw one of them
Ghostform - Smuggle two guys once
Dimir Machinations - Topdeck manipulation that can be used as a tutor instead
Dream Cache - Slow brainstorm that can put the cards on the bottom to avoid getting Brainstormlocked
Never // Return - Sorcery speed Hero's Downfall with a second side for bigger hits with Yuriko
Connive // Concoct - Topdeck manipulation and reanimation, with two sides for huge Yuriko hits
$ Dauthi Embrace - Enchantment that lets me smuggle any number of dudes each turn, as long as I have enough black mana
Death Match - Removal every turn I ninjutsu, but puts my ninjas at risk
Distortion Strike - Smuggle twice, but less flexible than Artful Dodge
Open into Wonder - Smuggle a bunch of dudes, but I'm probably not gonna have that anyway
Phyrexian Splicer - Steal evasion, but does nothing if your opponents don't have the appropriate keywords
Heirloom Blade - Ninjas replace themselves when they die
The Cauldron of Eternity - Hits like a truck with Yuriko, and lets me get back some dudes in the late game
Haunted Crossroads - Get back creatures and guarantee a certain amount of damage from Yuriko
Rags // Riches - Wipe small dudes, then get more dudes, and deal loads of damage if revealed by Yuriko
Nephalia Academy - Discard to top of deck
Sanctum of Eternity - Save Yuriko, or bounce to re-use

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Considered cuts:
Sensei's Divining Top - Needed in Narset, and it doesn't really do that much here anyway except when I only have a single ninja
Augury Owl - Cheap evasive creature with topdeck manipulation on ETB, but is it enough?
Invisible Stalker - Cheap, unblockable and hexproof, but still a pretty bad topdeck
* Far // Away - Decently big Yuriko damage with good utility, but both parts might be too small to be worth it
* Run Away Together - Bounce something of theirs to save or reuse a thing, or just two creatures from opponents
* Crystal Shard - Mostly here for Etrata, but can also bouncce a ninja for later ninjutsu, or keep people from tapping out
Scheming Symmetry - Might not want to give away a tutor
* Phantom Ninja - 3 mana with no ninjutsu and no protection might be too much to ask in a high power meta
Temporal Mastery - With a single Brainstorm as my only way to put this back on top if I draw it without the miracle option, this might be a bit hard to play
* Rogue's Passage - I can't afford a lot of colorless lands if I want a turn 2 Yuriko, and 5 total mana to smuggle a single creature is too much considering the amount of evasive creatures I play anyway
* Azra Smokeshaper - Only being able to ninjutsu in combat makes the flexibility of the ETB trigger not great, but it is a cheap ninjutsu cost
Dimir Infiltrator - I don't have that many important 2-drops. Becomes very important if I play Scroll Rack

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Cuts:
Mind Melter, Ferropede - Unblockable and has an extra ability, but still pretty bad topdecks
Gate Colossus - Can be hard to block, and can put itself on top of library for Yuriko triggers, but I can never play more than two gates, so I can't make this particularly cheap or recur it a lot of times
Mistford River Turtle - Can only smuggle non-humans, and has to attack while putting itself at risk to do so
Elusive Tormentor - Too much work to get an unblockable dude if I'm just gonna bounce it anyway. Could be nice to flip in response to a board wipe, but probably not worth it
Dream Prowler, Hazy Homunculus - If I'm gonna play a "vanilla" unblockable, I might as well play an unconditional one for 1 mana
Nevermaker - Currently played in Roon who can trigger it more often, but here it can be used to put one of my own things on top for Yuriko damage
Rite of Undoing - One of worst of the playable delve spells, and we don't want to overload on that
Set Adrift - Another delve spell, and the sorcery speed makes it almost unplayable
Library of Leng - If I need to discard (e.g. looting), I can plan a big hit from Yuriko, but this seems too narrow
Toothy - Drawing cards when ninjutsued, but accumulating the counters could be slow
Dire Undercurrents - Can probably trigger this once per turn, but that just doesn't seem good enough
Aqueous Form - Great to put on Yuriko, but it's not worth it on a dude that I'm just going to ninjitsu
Whirler Rogue - Nice to get new tokens every time this is reused, but is it what I want to spend 4 mana on every turn?
Marang River Prowler - I probably don't have enough black permanents to recast this, and 3 mana for an unblockable is a bit expensive
Archaeomancer - Synergizes with both ninjutsu and bounce spells, but it's 4 mana
Cavalier of Gales - Brainstorming is nice in the deck, but these creatures might be too expensive to keep replaying
Tasigur's Cruelty - Too many delve spells, and this is the worst one
Graveshifter - It's a ninja, but an expensive one without ninjutsu
Leyline of Anticipation - It's nice to flash in dorks at end step to give them pseudo-haste, but this is a 4-mana "do nothing" in most other cases
Veil of Secrecy - Save something from removal and/or smuggle it through
Key to the City - I probably won't have a lot of mana spare to pay to recoup my card, and playing a unblockable dork is probably better
Manifold Key - Only nets mana with Sol Ring, so this is basically a 3 mana + tap to make a single thing unblockable
Mortuary Mire - Always comes in tapped
Vela, the Night Clad - The intimidate is nice, but might not be enough and the other effect is minor
Whispersilk Cloak - Expensive to equip if I'm just going to bounce the equipped creature, but it does protect and continously smuggle an on-board threat
Mutavault - Selling merfolk
Dimir Locket - Low curve and lots of card advantage, probably don't need this
Exquisite Blood - Can gain a bunch of life off Yuriko triggers, but 5 mana is a lot for something that might not do anything
Terramorphic Expanse, Evolving Wilds - Slow "fixing"
Bojuka Bog - Too slow, and I'm probably fast enough that I don't need this anyway
Creeping Tar Pit - Enters tapped, and 3 mana to activate is quite a bit, especially if I'm just going to bounce it and have to replay it tapped
Murderous Cut - I just can't support this many delve spells
Throatseeker - Lifelink on anemic beats is probably not going to keep me alive through more than a slight breeze, and this can't be ninjutsued even though it is a ninja
Coastal Breach - Even with the discount, this is going to cost 4 or more. Better used in Noyan Dar
Etrata - 4 mana is a lot to replay, and I might not have time to kill people with the ability
Dimir Keyrune - Very low "real" curve, so might not need a lot of rocks, especially at 3 mana
Consign // Oblivion - Tempo or protection with a theoretical bit of extra value on the back end, and domes for quite a bit with Yuriko, but I'm really never going to play Oblivion
Commit // Memory - 4 mana for a soft counter is a lot, and 7 for a wheel that requires me to first cast the 4 mana spell is definitely too much considering the amount of cards Yuriko will give me, so this is just a burn spell
Wayfarer's Bauble - I want to play a dude on turn one, and then I only really need the second land to start drawing, which this doesn't help me with
Gudul Lurker - Replaced with a cheap ninja in Mothdust