Basically this was based off art by @sm0lb3tch (so credits again to them) and here we're going to make ObaMitsu canon, give the Kamado siblings new parents, give Muzan an aneurysm trying to find Nezuko when her whole family is a bunch of Hashira and as you can probably guess - deep breath - FOUND FAMILY TROPE.
Obviously in order for Obanai and Mitsuri to be the ones saving Tanjiro and Nezuko that fateful day we'd need to have them on the mission instead of Giyuu. For the sake of this theory Kagaya has assigned Mitsuri to accompany Giyuu, causing a very jealous and fuming Obanai to request (demand) to go in Tomioka’s place instead. Whether or not Kagaya is actually a shipper is up to you, but for whatever reason he says yes and Obanai proceeds to leave for the mission with Mitsuri a day or two earlier because he doesn't want Giyuu to argue with the decision when he finds out (he wouldn't) and wants some extra time with a certain pink haired crush. Mitsuri doesn't mind, she's more than happy a dear friend is accompanying her, after all!
When they reach the village Mitsuri’s a little embarrassed that she ordered so many helpings that the ramen store owner had to go out and buy more charcoal for the stove from the seller outside and Obanai's rather put out that the inn is too full for the both of them, not wanting Mitsuri to go camp out in the cold like she had suggested. Even more put out, in fact, with all the attention she's showering the charcoal seller, gushing about how he reminds her of her own brother and how hardworking he was to be doing this for his family - some idiot kid named Tanjiro something. At this point Obanai is having an existential crisis torn between disliking Tanjiro because Mitsuri's so nice to him (even feeding him a bite like in the Hashira Training Arc) and being nice to Tanjiro as well because whatever makes Mitsuri happy, makes him a happy camper. So he just settles for cold indifference.
(Yes, Tanjiro knows he's mad but doesn't realize that Obanai is mad AT HIM).
Anyways when Mitsuri expresses their problem of having nowhere to stay Tanjiro excitedly chirps that his family would be more than happy to have them over. Mitsuri, not wanting to be a bother, tries to refuse but Tanjiro insists so they compromise by letting her pay.
Then Obanai insists on paying but anyways-
We all know what happens from here so I won't go into too many details lest I bore you (small note that Tanjiro does not stay at his neighbour’s house this time and the timing is a little different to fit) but even Obanai felt terrible when they arrived and Tanjiro found his family shredded and dead. Since none of them realized Nezuko was a demon, Mitsuri helped Tanjiro begin carrying her down the mountain in order to seek help.
However the moment the truth was revealed Obanai immediately wanted to kill Nezuko despite Tanjiro’s pleas. Chaos ensues as is canon until an unsure-on-what-to-do Mitsuri finally succeeds in separating them when Nezuko suddenly defends her elder brother.
Mitsuri pleads for Obanai to reconsider, backed up by Tanjiro who says there's another demon's scent in the house so it wasn't Nezuko, at the very least take Nezuko to Oyakata-sama? If it was anyone else Obanai would have probably shoved aside to stab the demon, but since it was Mitsuri asking after all and as much as he hated to admit it he knew what it was like to have your entire family killed in such a bloody manner - so he agreed and we all know how the Hashira meeting went down.
I think I better clear up some points here, so because we have no very convincing letter from Lawyer Urokodaki to back Nezuko up we'll have Mitsuri testify that despite Obanai attacking her Nezuko made no attempt to eat him and only did a little to protect Tanjiro as well as a very grudging witness of a Serpent Pillar and why am I writing this like a court case-
Here's another important factor to how Nezuko gets off. In canon everyone dislikes (some of them at any rate) Giyuu and were very eager to have him punished and Nezuko killed, but here we have Mitsuri and Obanai as the defendants- uh, Hashira involved. Mitsuri is popular, so I feel like this would make everyone else more sympathetic to letting Nezuko live or just consider it. Obanai, not as much, but seeing how as Sanemi was the main aggressor in the canon events, I imagine he’d be pretty reluctant to throw his friend to the wolves as much as he hates demons. Also the journey to the Ubuyashiki House and summoning all Hashira would’ve taken time: time that would prove that Nezuko sleeps instead of consuming humans.
So, Nezuko and Tanjiro get off but Kagaya requests that they both become slayers to prove themselves and yadda yadda yadda. The catch is, he’s assigning Mitsuri and Obanai to do it.
(Once again we know not if Kagaya is secretly a shipper, but he said it was because a Hashira would be able to protect them both better and I don’t think any cultivator would want them.)
Mitsuri is ecstatic! Having the Kamado siblings live with her in the Love Estate is like having her younger siblings around again! You best believe this woman is treating them to six course meals (she eats twelve), shopping trips and really loves it when they come into her room for a sleepover to the point Nezuko and Tanjiro are faking reasons to just go in. A few “nightmares”, nothing much~~
Obanai, on the other hand…he’s definitely pissed off. Extremely so. But he wouldn’t disobey Kagaya and so grudgingly agrees to train Tanjiro. He’s as harsh as he was in canon, but I think at some point Obanai isn’t going to stay petty and jealous forever, and eventually warm up to Tanjiro and Nezuko’s sunny nature, probably after realizing Mitsuri just sees Tanjiro as a brother, Nezuko isn’t stealing ALL her attention, they could still hang out together alone and those precious moments he got to spend with Mitsuri a lot more.
”Aw, Obanai, that’s so sweet of you!” Mitsuri clapped her hands together, beaming at him. “I had no idea you cared about Nezuko so much!”
“…yeah, well, don’t mention it.” Thankfully his bandages covered his reddening face as he put away the wooden swords. What was Mitsuri so excited about? All he had done was to drape his haori on the sleeping demon when she suddenly dozed off in the midst of watching yet another brutal training session between him and her brother - a brother who was now being forced to run twenty laps around the Estate for even mentioning his actions.
“I know, but I can’t help it! You’re really a softie inside no matter what you say to Tanjiro!”
“What? Since when?!”
Mitsuri laughed at Obanai’s shocked expression. “Like yesterday! Tanjiro’s food was cold but he didn’t want to make a fuss at the restaurant but you went and demanded another plate, or when Nezuko shrunk and hid in my house you came over just to help us find her. Then it turned out she got stuck in one of the cupboards and you helped dig her out. Plus you’re always so nice to me too, like when you got me the socks, or offer to pay every time we go out to eat-”
“Okay! Okay! I get it, please stop, Kanroji…” Kaburamaru hissed reassuringly as Obanai quickly turned away from Mitsuri to hide his red face.
“You can call me Mitsuri, you know?” Mitsuri suddenly yanked him into a crushing hug excitedly, oblivious to Obanai’s flustered state. “Kyaaaaaaah! I’m so glad Oyakata-sama assigned us both together to train and take care of Tanjiro and Nezuko! I’ve gotten to spend so much time with my favorite Hashira - uh, wait, that’s Shinobu…so male Hashira! - and you’re actually really good with kids! I think you’ll be an amazing dad if we ever have our own kids one day!”
Silence.
“…I’MSOSORRYI’LLGOCRAWLINTOAHOLETOHIDENOW-”
“….that’s…a…nice….thought…”
“AAH, OBANAI, I MADE YOU FAINT I’M SO SORRY-”
Well, you can pretty much guess how they got together after that.
Because I’m a human and at the end of the day I’m lazy I will not elaborate and instead let your imagination run wild with how exactly they officially confessed after that night, broke it to the Kamado siblings and the rest of the Hashira and the messy details I can’t think of right now. I will, however, talk a bit more on how it changes the storyline.
Obanai eventually moves into Mitsuri’s Estate, shortly before the Final Selection (they celebrate with pancakes when Tanjiro comes back safely by the way; Obanai had a hell of a day trying to calm Mitsuri after she couldn’t stop crying worried that Tanjiro would be eaten. Now that I think of it, this might cause them both to go lodge a complaint on how the Final Selection is being run, because how exactly sending kids off to certain doom constitute as teaching? They should’ve done some other tests, one where they could monitor and guide where they did wrong and y’know, NOT MAKE ANOTHER GIYUU? (Sorry, I’m getting off track)).
There won’t be much variation for the beginning of the storyline, save for the fact Tanjiro and Nezuko have a home to return to in between for Mitsuri to cheer that they made it back and spoil while Obanai berates them for taking so long and fixes their ruined haori (I have like this idea that Obanai struggles with love language so he mainly sticks to small gestures like gifts revolving around clothes because you’ll use them every day and I dunno, if he was stuck in that cage in a house full of women he might’ve picked up a bit of sewing and figure out how to fix the holes in his loved ones’ haori?) as well as Tanjiro’s relationships with the Hashira change:
Obanai and Mitsuri aside, a major difference would be Giyuu. Tanjiro would not be developing a bond with him in this theory and honestly I feel like he would be the one to heavily advise against allowing Nezuko to live, therefore creating an enemy in Tanjiro. It doesn’t really help that’s he’s a loner whose words really don’t make sense to anyone and Obanai hates his guts, so it’ll take longer for Tanjiro to crack open his backstory and he would probably treat Giyuu like how he behaved to Sanemi in the Training Arc - “It’s okay! I don’t acknowledge you either since you didn’t want Nezuko to live.”
I’ll just skim through the rest of the Hashira before we move on. Shinobu, Rengoku, Sanemi and Muichiro would be a heck lot closer to Tanjiro and Nezuko since the four of them are on good terms with their adoptive parents, hence the Kamados would be seeing Shinobu a lot more often, cry harder at Rengoku’s death, soften Sanemi (perhaps fix his relationship with Genya?) and transform Muichiro into the silly boy we love earlier.
Do note, however, in this theory Tanjiro has mastered not one but TWO Breathing Styles and I’m sure he would then pick up Total Concentration Breathing a lot faster, so he’s definitely even more powerful in this universe. The same would apply for Nezuko, though in the sense that her physical prowess is better having been trained by two Hashira because even Mitsuri and Obanai can’t do anything about Blood Demon Art. Although, at the end of the storyline Tanjiro still learns Sun Breathing/Hinokami Kagura.
Another thing to be noted is that while I said MUCH variation that doesn’t mean there won’t be any subtle ones in Mitsuri and Obanai’s behavior and personality. I can’t really think of much for Mitsuri, other than the fact she’ll be more motherly and stuff, and probably really ecstatic that she’s finally achieved her dream of finding someone who’ll love her unconditionally and a family. I can see Tanjiro and Nezuko being really popular with her siblings as well.
For Obanai there’s a much bigger gap, as he’ll definitely mellow out and be less snarky. You know those gruff, serious dads who don’t show emotions or love easily but if you make a dumb crayon drawing from kindergarten he hasn’t taken it down from the fridge since and if some bully even implied you’re stupid they’re a goner because he’s there? Something like that for Obanai, I guess. Basically he’d be a lot less sarcastic and cutting with others and not as jealous because he knows Mitsuri is his and well…he’s not gonna admit it but he’s actually very touched when Tanjiro said he’s really glad he’s his new dad (Uncle Sanemi and Godbrother Muichiro anyone-).
BUT BUT BUT the big difference arrives on Mount Natagumo: home of Rui.
Instead of Giyuu and Shinobu arriving it’s ObaMitsu because what parent doesn’t worry about their kids, especially when they hear it’s a Lower Moon. So in this case it’s Mitsuri off to go save Zenitsu with Nezuko, whom Tanjiro had ordered to go help their surrogate mom navigate the crazy mountain and find Spider Sister, although Mitsuri is going to kill her with a lot more compassion…and leave Tanjiro to attempt to get Obanai off from stepping on the remains of Rui.
Okay, serious question, do you think Obanai would stomp on them more to be petty-
(DON’T KILL ME, I’M SORRY, I LOVE RUI AS MUCH AS THE NEXT PERSON, IF NOT MORE.)
Back to the storyline, this time round the reason Tanjiro and Nezuko are taken to yet another Hashira meeting is due to being able to celebrate the success of having finally slain one of the Twelve Kizuki. They still will go to the Butterfly Mansion, of course, most probably because their adoptive parents have missions of their own and honestly do you really trust them to be your doctor?
Anyways, aside from the little differences like it’s Mitsuri who brings her adoptive kids to the Swordsmith Village and Obanai going to the Entertainment District to go pick up his kids it’s all yadda yadda yadda, so we can pretty much guess what happens next, therefore I’ll jump straight to the two endings/outcomes that would be changed because of this theory. Depending on your mood today you can read the Bad Ending or jump straight to the Good Ending and don’t say I didn’t warn you <333 (I gotta go hide before you hunt me for the Bad Ending, ta!).
BAD ENDING:
Notice how not once did I say that Obanai and Mitsuri ever got married though?
Bwhahaha. Now I am. And you’re not going to like it.
Obanai had the ring (helped picked out by Nezuko). He had the plans (dinner made by Tanjiro; Shinobu would be babysitting the kids that night). He wanted it to be memorable when he would finally ask Mitsuri to allow him the privilege of becoming a Kanroji and probably officiate the adoption of Tanjiro and Nezuko.
But it all got shoved aside when they had to do the Hashira Training. Obanai slowly regrets waiting that long, but while he’s not an optimistic person in general he hopes that afterwards he’ll get the chance when everything is so perfectly in order. Of course Muzan took one look and thought “absolutely not” - and deposits them all into the Infinity Castle.
So it’s only in their dying moments that Obanai confesses. He can’t even put a ring on her, because she’s lost her arms.
Mitsuri said yes and asks if they’re reborn would he still marry her?
He says yes. At least they die happy. Unable to be pulled apart even in their last moments.
(Tanjiro and Nezuko ensure to carve a love poem into their headstones and bury them in wedding clothes, so at least they can be together in death.)
GOOD ENDING:
Now that Obanai and Mitsuri have SOMETHING to live here they’ll live on in this theory. Mitsuri armless and Obanai totally blind, but hey, with diligent kids like Tanjiro and Nezuko they would be well taken care of no matter what, rebuilding their family slowly.
Eventually Obanai and Mitsuri are blessed with their own kids! Five, if I remember what happened in canon, so because I’m self-indulgent let’s simultaneously rebuild the Kamado family and combine the OC children I originally made for them. Nezuko’s delighted to receive two twin younger sisters and a baby one, spending majority of her time with them doing their hair or just playing dolls (Zenitsu steaming mad because they keep stealing her attention and don’t like him). Tanjiro feels like he’s got his own siblings back again with the Kanroji’s two rowdy boys who often beg him to teach them swordsmanship and perform the Hinokami Kagura. Does Obanai gift them all tiny snakelings? Absolutely. Does any of them inherit Mitsuri’s hair? One or two, but Mitsuri’s sakura mochi is a family recipe now (I’d like to think Kotetsu somehow managed to forge a pair of mechanical arms for her).
Oh, imagine Mitsuri and Obanai at Tanjiro/Kanao and Nezuko/Zenitsu’s weddings! Mitsuri would be amazing mother-in-law for anyone and you bet your ass that she helped plan the best wedding possible for them, welcoming Kanao and Zenitsu happily into the family and perhaps being a little embarrassing for the newlyweds because she’s asking when they plan to have children or a honeymoon - sorry, she can’t help it, she’s just so excited! Obanai dislikes Zenitsu INTENSELY (he dislikes all boys near his girls, but Zenitsu the most funny enough) and makes it VERY known with glares and snide remarks; he likes Kanao better though, so he’d probably tell Tanjiro to treat her right or he’ll give him a walloping blind or not.
AND WHEN THE KIDS COME-
(exhales, clears throat, reminds myself that other people are watching) So I hope YOU cried, because I did. And I’m not totally cruel, alright! So for funsies here’s an extra snippet in which Mitsuri and Obanai reincarnate as the godparents of Tanjiro and Nezuko’s descendants:
“Are you-” The man ran his hand through his already unruly ponytail again. “Are you actually stupid? You better be. You have to be. You can’t possibly not be.”
“I’M SORRY! I REALLY AM! I DIDN’T MEAN TO POUR IN SALT INSTEAD OF SUGAR! TOKO, HELP ME!” Yoshiteru was a snotty, emotional mess as always, even more highly strung when faced with his godfather Obanai’s cold rage…especially after he made a mess of himself as usual.
“I CAN’T BELIEVE YOU’RE STILL ASKING ME FOR HELP AFTER THAT!” Toko smacked her brother upside on the hand. “Don’t even associate with me right now! The cake must be disgusting, and Tojuro can only distract Sumihiko for so long before he comes to the restaurant!”
Obanai pinched the bridge of his nose, letting out another exasperated sigh. “We’re going to have to start all over again; Yoshiteru, get out of the kitchen and if I see you so much as pop your head by the door or so help me-”
“Hey, hey, Obanai, you don’t have to be so harsh! It was a genuine mistake after all!”
The pink-haired woman pulled her husband into a quick hug, pressing a quick peck to his temple that left him tomato red under his face mask. Kanata followed from behind, holding a tray of fresh baked goods and obviously bewildered by what was happening.
“Mitsuri - I told you I could handle this - you need to rest-”
“What, sit back and do nothing for my godson’s birthday?” Mitsuri pouted, rubbing her swollen belly. “I’m sure baby wants to help as well!”
“SEE? IT’S NOT SO BAD!” Yoshiteru hollered, desperately trying to escape his sister’s glowering. “Tojuro’s good, he’ll probably keep Sumihiko occupied for a really long time!”
“Oh my god, just get out of the kitchen and go help Mom and Dad set up the restaurant for his birthday party or something!” Toko gave him a powerful kick out the door.
“HEY! Aren’t the Agatsuma girls supposed to be sweet and gentle? You’re a monster!”
“A monster who can bake a cake without creating disasters!” Obanai barked.
Mitsuri laughed. Obanai could never get enough of that sound. “You’re going to be a great dad…”
“…thank you…you’ll…you’ll be a great mom…too…”
“Sorry to interrupt Iguro-san fainting again but Tojuro just called and said Sumihiko is coming back,” Kanata said matter-of-factly, shutting off his pinging phone.
“WHAT?! TOJURO THE TRAITOR-”
Yoshiteru seemed to have awakened Obanai from his near faint. “What did I say about coming back into the kitchen?!”
