12(Demon Slayer)

An Au where the Tomioka and Agatsuma clans are known for creating/experimenting with a variety of miracle cures and dangerous poisons which comes since Muzan's era(but they have no connection with Muzan's doctor...said doctor just wanted to be more famous then both clans but it backfired).

During the Tsukiguni's era, the Tomioka and Agatsuma dwindled as Muzan killed them after they refused to tell him were the Blue Spider Lily was until only five of each clan was left behind.

Tomioka Yuu and Agatsuma Zenko were two of the surviving members of the clans but unfortunately they found themselves kidnapped and sold in the Red District, they thought they would find themselves violated until their last breath and have their clans completely die out but they they were saved by Tsukiguni Michikatsu who bought them from the man/house they were sold to and took them as his concubines with the permission of his wife(who took great care of them since she knew Michikatsu wasn't attracted to them and he treated them as his sisters/daughters).

Yuu and Zenko were basically like children to Michikatsu and saw his wife as a second mother and their children as their sibling so when Michikatsu suddenly decided to leave they knew something bad would happen especially as he promised to come back and never did.

Seeing this, Michikatsu's wife had to find good husbands for the girls who would not mind taking their names as their own instead of keeping their own especially as while Michikatsu's wife was safe as she still had her son to protect her, the two girls didn't have it.

Yuu and Zen marry two kind man and decided to continue passing down the work of their clans but also make sure their descendants known about Michikatsu and his kindness which they still have to pay for.

Many years later Tomioka Giyuu and Agatsuma Zenitsu are the last surviving members of the clans with only books full of their clans story and learnings(which includes were the Blue Spider Lilies are) to their name which they never give to anyone and when they run away/leave, they take with them(here Zenitsu's parents died when he was 4 and he went to live with his aunt, who was the one that kicked him out later).

Things are mostly canon except that Giyuu and Zenitsu unknown to almost everyone practice their ancestor's teachings and create their own cures and poisons(maybe they end up working together to find a cure for Nezuko?) and that Giyuu took Muichiro under his wing as he knew instantly that he was Michikatsu's descendant.

It isn't until Giyuu visits Tanjiro in the Butterfly Mansion(before the mission with Rengoku), that Giyuu and Zenitsu find out that there is someone from the other clan as their eyes are something only found in their clans, of course that they aren't able to talk much at the time but they are thankful to have someone who understood why they kept an act and hide the truth of their ancestors.

During the fight with Akaza, Zenitsu somehow is able to affect him with one of the poisons he made so Akaza doesn't end up killing Rengoku but does injure him gravely(which is resolved with a cure Tomioka uses when he sneaks into Rengoku's room at Butterfly Mansion).

Basically, Zenitsu and Tomioka have been working in the shadows in helping the Demon Slayer Corps, including Tomioka giving a medicine to Oyakata that made his curse progress much slower then it did in canon(so he isn't completely blind yet)..

The only one that has a inkling of something is Sanemi who had heard about the Tomioka family miraculous cures in his village but he never said anything as he owed the Tomiokas for not helping saving his abusive father(so he won't be as antagonistic towards Giyuu but he is still very hot tempered as in canon).

About the Michikatsu storyline in this idea...I believe one of them would meet him before the final fight and make the connection between Kokushibo and Michikatsu besides Kokushibo would freeze when he saw Giyuu and Zenitsu but wouldn't be able to tell why they seemed familiar.

So Zenitsu and Giyuu would be fighting Kokushibo instead of Sanemi and Genya, and they would be the ones to kill him by using a poison/cure that would make him human again but his body match the age he is(so he would technically die of old age), they would thank him for saving their ancestors and pray he would meet them, his wife and child soon while Michikatsu would probably say how much they look like Yuu and Zenko or something like that.

Anything else is up to the writer.