16(Demon Slayer)

An Au where Iguro is a women and was saved by fem Urokodaki who took her in and raised her alongside her three students(Makomo, Sabito and Giyuu).

At first Iguro only got along with Urokodaki and Makomo as she feared men due to what happened to her before she was saved by Urokodaki but Giyuu with his quietness and subtle kindness despite being awkward got her to start open up to him and later Sabito too.

Urokodaki and her two eldest students were witnesses to how Giyuu and Iguro slowly grew close and fell in love.

When Sabito and Giyuu were 14(a year later then canon), the four kids went to the Final Selection which despite Urokodaki's fear of losing them, they all came back alive even if Sabito and Makomo ended up losing a arm(Sabito) and a leg(Makomo) because of the demon who hated Urokodaki(here, the demon was killed by the work of the four together).

Giyuu and Iguro decided to still be demon slayers while their friends decided to join the Butterfly Mansion(Makomo) and help Urokodaki with any other students he may gain(Sabito).

Giyuu officially started courting Iguro when they were 16 and at 18 they got married with only having their sensei, friends, Oyakata and his family present as they saw no reason to make it something really big but Giyuu made it sure it would be a wedding his sister would be happy with as he wanted to have the wedding she never got to have.

Both Giyuu and Iguro still became hashiras at the time they did in canon, Iguro became one later as she decided to create her own breathings style since she felt that the Water Breathing Style didn't match her well.

One time when they were visiting Urokodaki and Sabito, they met Urokodaki's old friend(Jigoro) and his two students who they saw a lot of themselves in.

Kaigaku reminded Iguro of herself and her family but mostly how he seemed to carry guilt and due to his fellow student he was trying to make up for what he did while Zenitsu reminded Giyuu of himself that hide his feelings behind an emotionless face(in Zenitsu's case in crying) but still tried to help Kaigaku on her own way.

(Here for Kaigaku's past it would probably mostly be the canon one but you can change it if you have a better fit but for Zenitsu I was thinking that she had been a very happy girl in her village until a boy abused her and everyone started treating her like dirt as they believed she seduced the boy but Jigoro and Kaigaku while passing her village saw how she was treated and Jigoro decided to take her in which her parents didn't mind as they demeaned her a disgrace to their family.)

So they decided to take the two of them under their wing with Kaigaku ending up creating a mixture between the Serpent Breathing and Thunder Breathing styles.

A few years of Giyuu being a hashira, Oyakata asked him and Iguro to take in and keep an eye on a boy they had recently saved as he had lost his twin and seemed focused in revenger despite his memory problems.

In the end the couple ended up becoming parental figures to Muichiro and helped him train while imposing limits so he wouldn't hurt himself. And despite not wanting him to become an hashira or demon slayer as early as Muichiro did, they were very proud of him and celebrated it with their small family(Sabito, Makomo, Urokodaki, Jigoro, Zenitsu and Kaigaku).

None of the other hashiras would know about Giyuu and Iguro being married and basically Muichiro's parents until during a meeting(after Tanjiro's appearance but before Rengoku's death...with probably the characters a bit aged up in some cases) which Iguro hadn0t arrived to yet, Zenitsu invades the meeting shouting for Giyuu.

Of course that Sanemi and a few others were about to reproach her until Zenitsu basically shouts that Iguro is having a panic attack after being told she was pregnant by Makomo.

Giyuu after hearing that, didn't lose time in getting up and running to his wife to help her calm down and Zenitsu followed him.

Seeing this the other hashiras looked at each other confused(if they also knew that Iguro was women is up to the writer) until Oyakata requested them to follow as he wanted to know if Iguro was fine and what decision she and Giyuu would take.

The reactions to the reveal and them discovering how little they knew about three of their hashiras and a few slayers/healer is up to the writer but I wanted Iguro and Giyuu to decide to keep the baby and that they would both survive the final battle with no demon slayer mark(but if they get one, that they leave their child in Sabito/Makomo or Kiagaku/Zenitsu's care when they die).