A/N: I still haven't finished watching Demon Slayer but I got this idea after seeing an image of a Madara fanart in demon slaying clothing. If you have knowledge on Demon Slayer then this could be a cool idea.
---
Madara Uchiha in Demon Slayer...
(Here is the pic)
Madara Uchiha and his little brother Izuna were orphans.
Madara will be the same age as Tanjiro.
They got orphaned due to their mother being sick which lead to her death after giving birth, and their father died before their birth.
He decided to become a demon slayer after he and his little brother Izuna got ambushed by a demon which resulted in Izuna's death. Making Madara the last living member of the Uchiha family.
You can give the Uchiha family a cool backstory and history that Madara later learns about (He won't know much due to being an orphan) You could even say that they went extinct due to being one of the families that have been fighting the Demons for generations but most ended up losing their lives. (One of the Uchiha ancestors could have been one of the people using the Breath of the Sun)
His goal is to achieve peace by getting rid of all of the Demons.
You wanna keep Madara's character appearance and personality close to the original in Naruto but this time he won't be a villain. So his cold personality and arrogance will still be there but he will be a good guy.
Izuna's death also lead him to awakening the Sharingan, a rare ability that an Uchiha family member awakens every few generations. The last being his Grandfather Tajima Uchiha.
---
This is where im lost tho...
I don't know who will teach him swordsman ship and a breathing style... Maybe an OC?
---
Then he goes to the final selection and passes, meets the main characters and interacts with them.
Follow the maim story line from here while adding some original plots etc... You could even add romance...