One great 'Muichiro x Reader' fanfiction is 'A Meeting in the Mist'. It beautifully captures Muichiro's somewhat aloof nature while gradually developing a sweet relationship with the reader. The descriptive language used really makes you feel like you're in the Demon Slayer world with them.
The relationship development between Muichiro and the reader has to be believable. It can't be too rushed. A slow - burn, where they gradually get to know each other, understand each other's quirks, and form a deep connection over time is much more appealing. Also, the setting should be in line with the Demon Slayer universe. Whether it's in the Demon Slayer Corps headquarters or on a mission, the details of the world should be well - described.
One popular 'zenitsu x nezuko fanfic' is 'The Unlikely Bond'. It beautifully portrays the relationship between Zenitsu and Nezuko. The author does a great job of showing Zenitsu's protective nature towards Nezuko and how Nezuko's presence calms Zenitsu in his often-fearful state.
Well, a lot of it has to do with the potential for character development. In the original story, their interactions are limited, but in fanfics, authors can explore how they would grow closer and influence each other. For example, Zenitsu could become more confident because of Nezuko, and Nezuko could learn more about the human side of relationships through Zenitsu. It's also about the fans' love for the two characters and their desire to see them in different scenarios together.