"Come on, go, reach for the stars, reach so high!"
As the above quote implies, I imagine DR execution music remixed with D4 music to play during everyone's execution. Saki is a blend of DR2 execution music and "4 Challenges". The rest will be listed below with descriptions of all 24 executions:
Rinku: World Tour: She wakes up onstage, expected to sing for a crowd of angry Monochios, but no matter what she sings, they boo and hiss. The execution is like Disney's "Carousel of Progress", spinning Rinku through stage after stage, but the result is always the same. The Monochios get up and walk out, and when none are left, Rinku is deemed a failure, and her concert is cancelled. The theater goes dark, then Monochio appears on a balcony above and opposite the stage. It gives her a thumbs down (claw down) and the theater doors burst open again, a lion patterned like Monochio quick to rush in and kill Rinku. (Plays with multiple Hapiara songs).
Maho: Maho's Hotpot: She is cooked alive (think the first part of Kiyo's execution). (Plays with "Direct Drive!").
Muni: Rabbit Season: She is hunted through the academy forest like a rabbit, suffering various injuries along the way. She is finally caught by a giant Monochio clone. Her rabbit ears are all that remain. (Plays with "Give Me Awesome").
Rei: Do-Re-Mi: Similar to Rinku, she has to perform increasingly complicated pieces for an increasingly irritated crowd, but what makes her execution different is that the piano is fighting her, lid banging open and shut, keys ignoring her touch, strings snapping and lashing around her... (Plays with "Brave New World/Happiest Dreams").
*Alt execution: Tea Time/the Bad Tea Party: When I gave Rei a Tea Ceremony Talent, I imagined her execution as one where she had to host a teaparty, but of course, it goes horribly awry as the guests wreak havoc of increasing severity. Rei is killed during the chaos, covered in tea and pink blood.
Kyoko: Reach for the Peak: She has to run and climb through an obstacle course to reach the peak of a mountain. There are checkpoints along the way where she has to perform a brief song against other faceless, robotic climbers. She loses every single checkpoint and it subsequently slows her down. Right as she reaches the peak, dead last, the stone beneath her hands and feet break. (Plays with "Let's Do the Big Bang").
Shinobu: Dance 4 Deadly Justice: She faces a round of four different games, Call of Duty, Mario Kart, Street Fighter, D4DJ. She plays on turntables while a Just Dance-styled avatar of Kyoko plays against her with a mic. (Plays with "Electric Chaos Countdown" or Chiaki's execution music).
*In my saga, Shinobu figures out how to hack out of her execution and does so solely for the purpose of luring Monochio into it so that she can trap it there when the timer runs out.
Yuka: Steel Nerves, Steel Cage: Yuka is thrown into a boxing ring where robots designed to look like Dalia and Ibuki battle her. She barely wins, then a muscled Monochio (looking like buff Kuma in V3) comes out and crushes her, literally and metaphorically. (Plays with "moment").
Esora: Caution, Construction Ahead: She starts on the ground with only a platform beneath her, but little construction bots swarm around her and answer her every beck and call. It's her job to instruct them well enough that they build a tower beneath her that lifts her up to freedom. Issues such as glitching workers and unstable construction material make the execution harder, and if Esora fails, the entire structure collapses. Think Jenga/Tetris. (Plays with "Peaky&Peaky").
Ibuki: Run Like the Wind: Ibuki is thrust into a maze, tasked with escaping before the monsters and traps get her, but she also has to lead frightened, robotic versions of her bandmates to freedom as well. Unable to balance keeping them calm, keeping them moving, and outrunning the dangers of the maze with everyone behind her, Ibuki meets her demise when she gets turned around and lot, unable to find her way out and unwilling to leave the bandmates she was separated from, even though they were only robots. (Plays with "Into the Storm").
Saki: D4V4 Blastoff!: Just the rocket execution. She's the 4th to endure it after Jin, Junko, and Kaito (Plays with "4 Challenges" or Kaito's execution music).
Noa: All the World's a Stage: She wakes up onstage with three black-clad witches chanting over a boiling pot. They're the past, present, and future versions of Noa and they force her into the pot. It's so hot that she's shot back out, landing on a balcony far above stage. The balcony collapses and she falls right in the middle of a war between the Montagues and Capulets. She is beheaded during the fight and her body performs Hamlet's soliloquy with her severed head. All the while, she's chained up like a puppet, forced through her execution from the very first step onstage. The chains are severed at the very end of the execution and Noa collapses to the floor, head rolling away into the darkness (Plays with "Photon Melodies/Here's the Light" or Leon's execution music).
Towa: Super Towa-chan Time!: She wakes up in what looks like her bedroom, except it's even bigger and grander and more decked out in all the things that she likes. At first, it seems like paradise, but escape from the room becomes difficult when merch and crepes keep appearing but Towa can't find the door anywhere. She is ultimately drowned by all of her clutter. (Plays with "Wonder Wonder Trip/What Are You?").
Rika: Floor Killer/Floor Burner: Similar to the scene in Petite Mix ep 19, Rika must make her way across a long, booby-trapped floor. She loses when she reaches a section where fire is shooting up from beneath the tiles and she eventually falls through. As the Ult. Arsonist, she survives in the fall and lives long enough to hallucinate her unit-mates in the flames before finally dying. (Plays with "Floor Killer").
Saori: Diary of Despair: She wakes up in a depressing bedroom, every inch of the walls covered in calligraphy that does nothing but insult her or talk about how much she hates herself. When she tries to escape, the posters fly off the wall and attack her. Although it occurs offscreen, after the papers swarm her and back her into a corner, the execution cuts to her being a jar of ink the same way Mondo randomly and suddenly became butter in his execution. (Plays with "NO-NO").
Marika: All that Glitters/Midas Mizushima: She's in the middle of a photoshoot and everything starts well, but she is quickly buried under more and more costumes, props, and makeup as more and more photographers demand more and more poses from her. It culminates in someone pouring solid, molten gold on her. (Plays with "round and round").
Dalia: I Will Never Die: Dalia is chased through a dojo by her fallen friends before the one that she killed drags her into a grave with them. (Plays with "I Will Never Die" or Kiyo's execution music).
Tsubaki: Similar to Ibuki, Tsubaki wakes up in a maze, although hers is made of blue roses and she has to find her way out using echolocation and song (think Marco Polo). Like Ibuki's maze, Tsubaki's contains traps, but they attack her voice (ex: the air becoming polluted so Tsubaki can't sing) and even though there are tools scattered around, Tsubaki never figures out how to work a single one and suffocates. (Plays with "Movement").
Aoi: To All My Adoring Fans: Aoi wakes up at a celebrity meet-n-greet with faceless robots even further than the eye can see lined up for her. Knowing that she's in the middle of an execution, Aoi tries to politely push them aside and promise to meet more of them later, but they chase after her and she is trampled in the ensuing chaos. (Think of a paparazzi chase, which becomes even more fitting after RONDO S:O even though I had this idea before that). (Plays with "Prayer[s]").
Hiiro: House of Cards: Hiiro wakes up in a house of playing and tarot cards, all sorts of other witchy and fortune-telling items scattered all around. She has to use the items to predict when and where the Wind of Fate will strike, and subsequently, which room of the house is safest to hide in (based on location in the house, and what talismans are in the room). The goal is to survive until the storm stops. (Plays with "ReTINA").
Nagisa: Super Sonic Speedster: Nagisa has to ride her motorcycle through a gauntlet of electric guitars playing so loudly that they can create sonic booms. Eventually, Nagisa is knocked off her motorcycle by one of these sonic attacks and the still-rising decibels eventually kill her, as escape is far slower without the motorcycle. (Plays with "Calendula").
Miyu: The Lord's Favor: Miyu consistently insists that her "Talent" is just a result of hard work and blessings from God, so her execution forces her to flee and fight an archangel. If unlucky, the angel kills Miyu. If lucky, Miyu figures out a way to evade the archangel and become an angel herself, which allows her to escape her execution entirely once she has wings. (Plays with "Happy Prince").
Haruna: After School Detention with the Chairman: Haruna wakes up reading the Bible to faceless, robotic versions of her classmates and they begin to throw things at her in response. The objects escalate in damage they can cause, and the Kurumiiko bots are finally the ones to kill Haruna by beating her to death with chairs. (Plays with "Certified Human" or Peko's execution music).
Kurumi: Shame on Me, the Joke's On You!: Kurumi wakes up in a giant warehouse filled to the brim with every type of prank or joke toy, trick, or tool one could even think of. She is tasked with using everything at her disposal to build a cunning, clever tool out of the warehouse that surprises them by helping her escape in a nonconventional and unexpected manner. Joke's on Kurumi, though, as the true way out was just to use a door in the hallway behind her (which she misses because it's slightly obscured and she's too focused on the pranking stuff) and no matter what she builds, it WILL backfire on her. (Plays with "I Shall Be a Cat").
Miiko: Happy Adventure Queen!'s Last Hike: As the first execution, Monochio drags her away by her throat with its tail, throwing her onto a landing strip and forcing her to outrun a plane. She escapes into the sea where she then has to flee a submarine. She is left in a forest full of cute, friendly animals, but they turn into zombies and eat her alive until nothing but bone remains. (Plays with "Adventure King" or Gundam's execution music).
Order of executions: Miiko as the first Blackened, Saki for breaking a rule in Chap 2, Haruna as the second Blackened, Noa as the third, Dalia as the fourth, Shinobu as the fifth, although only because she volunteered to be executed in Kyoko's place since Kyoko was already dead by that point.