While I can't imagine Blair, Fiona and Natalia pulling off a dynamic like the Huntrix's in the KPDH AU (it's why they're sorted into mix gendered groups instead), I could definitely see them pulling off the kamoboko trio's dynamic in a Demonslayer or KNY AU.
Blair is definitely Tanjiro, carrying around a demonized Blake who is Nezuko. Her brother broke his leg that day so she had to be the one to go down to town to sell coal. She comes back seeing her parents are dead and Blake turned into a demon... They're just cursed to be orphans in every au. Vincent, a pillar, is the one who found the twins and much like with Giyuu, he was eventually convinced to spare Blake, seeing how protective he is of his sister despite becoming a demon. Blair pretty much follows along Tanjiro's story but I think she'd be pretty interested in picking up all kinds of tricks from other pillars or comrades. Definitely a fan of poison and is curious about guns. Gurl gets into an internal conflict about wanting to go back to a normal life vs escapism as a demon slayer. Like, she still desperately wants to get her bro back to normal but she isn't sure if she wants to go back to a normal life after training as a demon slayer. She likes being a demon slayer. In this era there wasn't a lot of career options for a woman besides being a good wife, which was ugh for her. Before all this went down she was pretty lost on what to do.
Natalia is like Zenitsu but with Shinobu's poison based breathing style. Her family was killed by demons too, leaving just her and her sisters. Velvet, her older sister, died as a pillar. She's just her tsugoku and not a very strong one. She does not have the physical strength to behead a demon. So she researched poison to make up for it. After her sister died she studied under the Demon Slayer Corps' doctor alongside her younger sister who she banned from demon slaying. The two argue about Natalia taking up the sword despite not being very strong and is actually pretty scared... But Natalia can't let go of her desire for vengeance and has given up so much for it like a chance at a normal life and getting married... That doesn't stop her from developing a crush on Blake though. It's only with him that she acts a bit more like a normal girl around her age.
Fiona is Inosuke in role only. She's just the third guy who butts heads with a cowardly demon simp. She's more of a Genya tbh. Her family were also attacked by demons except hers did survive. Her father moved to the blacksmith village and is always innovating weapons for the demon slayers. Her mom is a retired pillar, lost an arm and had to quit and currently lives with her husband in that blacksmith village. Her brother is the current ice breathing pillar. She has a deep admiration for all demon slayers for saving her family in time. And a deep hatred for demons, it's why she was gonna attack Blake on sight but Natalia kept getting in the way. She's still childhood friends with Keagan and Shizuke though they're all on separate fields... Keagan is gonna be the next head of the corps and is cursed to die young. Fiona's goal is to get strong enough and to end Muzan to break the curse on Keagan and his family as her way of showing gratitude for saving hers. Unlike most demon slayers, Fiona goes through a wide variety of weapons designed by her father. A gun is her constant back up.
Blake is Nezuko, though his demon form is more kitsune inspired than oni inspired. His demon arts is illusion based. He does regain his speech faster than Nezuko though he more or less relies on illusions to speak rather than physically speaking himself. At night, he can disguise himself as a human and can pass himself off as one so naturally, only demon slayers could really tell he's a demon. His brain is constantly foggy and has trouble remembering things instead of mentally regressing into a child. He only remembers that Blair was someone important to him so he follows her around and protects her. Natalia... She was just this weirdo hanging around Blair at first. She was scared of him and there's this sense of anger at seeing him but Blair said they should try to get along so he tried his best. He gave her presents, said nice things to her, taking her to festivals, showing up in his human form cause she seems more comfortable with that illusion, etc. I just wanted the oc equivalent for Zennezu to be a little more palatable, it was kinda weird when Nezuko spends most of the series mentally a child.
Vincent is pretty much Giyuu, the pillar that spared the twins and took them to his master/mother. He's considered the oddball of the pillars... Personality wise he's more like Rengoku. Cheerful but he hides how much he mourns every comrade he's ever lost. Especially his dad, who died to a demon attack. A bit of a loner, but the twins didn't think he'd be one since he's so perky around them.
Erin is basically Giyuu's master, the retired pillar that took a chance and spared the demonized sibling. She's cold and stern at first, really went all out on training to discourage Blair. She only really agreed to take them both in to keep a close eye on Blake. But in two or three years of being under her roof, not once did Blake gave in and attacked anyone. It took a long while to get her on their side but now she's their most consistent supporter alongside her son Vincent and Blair hates to admit it but she sees Erin as a parental figure.
Shizuke is pretty much Aoi but with Kanao's role as a potential love interest. He used to be a demon slayer but has quit and studied as a doctor. He was traumatized when the last group he was in all died except for him. He blames himself for not being able to save anyone. Not helped that some people resents that he of all people survived when there were more talented people in that group that deserved to live more. They did eventually apologize after Fiona confronted them and Keagan told them off, but Shizuke took it to heart. He was lost on his purpose for a while but he didn't want to stay idle and chose to keep helping in the hospital. That's where he first met Blair. Just like him, she was lost on her purpose as well and they kinda bonded over that. Blair did say though, his general direction seems to have always been wanting to help others. In the finale, he does end up doing what Kanao did but like, he was only there as a field medic. It's up in the air if he ever got a fight scene. He's pretty good at making wire based traps to slow down demons.
Keagan is the next head of the demon slayer corps and is similarly cursed like the one in canon. He's been delaying getting into an arranged marriage though he did think of asking Fiona. Too bad the girl was too determined to be a demon slayer that he decided against it, unaware she's doing it all to save him. His father has been in critical condition for a while so Keagan's pretty much taken over his duties. He's a great tactician and coordinated the demon slayers in the finale.
Damien is one of the pillars and has an intense grudge against demons. He lost his little brother to them. Definitely pushed for executing Blake on the spot. He and Natalia are still close friends. They came from the same village and lost their family to the same demon, who turned out to be an upper moon. They teamed up against that same demon in the finale.
Travis is basically the sound pillar. He used to be a ninja. He doesn't have a harem, just one wife and a child. Liera is the wife that went missing in the red lights district and it's why he asked around for female demon slayers to come with him and infiltrate to find her. Zephyr, his only son and tsugoku, snuck along much to his annoyance.
Zephyr pretty much inherited the title of sound pillar once his dad lost a hand and was forced to retire. Either that or Travis dies. Often tag teams with Damien.