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.
One common theme is the deep emotional bond between them. Given their relationship in the original work, fanfictions often explore how they support and care for each other on a more profound level. Another theme could be adventure, where they go on quests together and their relationship develops during those experiences.
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.
Muichiro's death in the manga occurs during a particularly intense confrontation. His injuries accumulate, and he ultimately succumbs despite his valiant efforts.
You can try fanfiction websites like Archive of Our Own (AO3). There are often a large number of fanfictions about various pairings, including muichiro x yuichiro. Another option is FanFiction.net, but the content quality may vary. You can also check out some dedicated anime or manga fan communities where fans might share their own written fanfictions or recommend good ones.