What if Yoriichi Tsugikuni and Michikatsu Tsugikuni (Kokushibo) swapped places with Tanjiro Kamado and Nezuko Kamado. They would swap places with Yoriichi taking Tanjiro's role and Michikatsu taking Nezuko's role.
Extra Background and rules.
Tanjiro was the Inventor of Sun breathing, but he was nowhere near as strong as Yoriichi.
Tanjiro didn't defeat Muzan and so never weakened him to such a degree that Yoriichi did.
Tamayo still broke free of Muzan's control.
Muzan is around 3 times as strong now, as he was in the canon story.
Muzan is also a lot more open killing more people and therefore adding to his strength.
Upper One has been replaced with someone else and without spoiling anything they are stronger than canon Kokushibo, just with less Battle IQ.
All The Upper moons are stronger since Muzan himself is stronger therefore giving them more blood.
Muzan is not as afraid of sun breathers, but is still wary of them. He hunts them down along with the Upper Moon One. This leads to leniency, and from that Sun Breathing is known, but very few live to actually document it.
Yoriichi and Michikatsu are a year older than Tanjiro at the start of the story, being 14 years old each at the start of the story.
Yoriichi and Michikatsu are from a different family (with the name Tsugikuni) Instead of Samurai, they come from a Demon Slayer family.
The background of the two are the same, except instead of Samurai stuff, it is Demon Slayer stuff.
Yoriichi is to be expelled from the house at 15 to go and work, which is why he leaves, but him being a prodigy changes that and he leaves to prevent his brother from being kicked out.
Muichiro is a distant cousin of the Tsugikuni brothers.
Now on with the Story.