One popular type might be the ones that focus on their rivalry turning into friendship and then something more. For example, there could be a fanfiction where they start off competing for a spot on the team but gradually realize they have a deeper connection. Another could be set in an AU (alternate universe) where they meet in a completely different context like at a music concert and their relationship blossoms from there.
Some Tumblr blogs also post or recommend Haikyuu fanfictions. You can search for Haikyuu fanfic blogs and then specifically look for Sakusa x Atsumu stories on those blogs. The bloggers often give a little summary or their thoughts on the fics, which can be helpful in deciding if it's something you'd like to read.
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 'opposites attract' concept. Given sakusa's quiet and clean - freak nature and atsumu's more energetic and sometimes messy personality, this contrast often forms the basis of their relationship in fanfictions. Another theme is their mutual growth. They help each other become better players and people in many stories.
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.