Giyu survives in the manga. The plot doesn't lead to his death. His character has many adventures and developments that keep him alive and active in the story.
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.
Their relationship can be developed through shared experiences. For example, if they both get injured in a battle and have to rely on each other for survival while waiting for help. This would force them to open up and communicate more, breaking down Giyu's usual stoic exterior and allowing Shinobu to see a more vulnerable side of him.
A plot could revolve around a mystery illness that is affecting the Demon Slayer Corps. Shinobu, being the expert on poisons and medicine, tries to find a cure. Giyu gets involved when he discovers that some of his comrades are showing symptoms. Together, they suspect that a new type of demon or a hidden enemy within the corps is causing this. They have to investigate, and during this process, they face many challenges and also grow closer as they share their concerns and ideas.
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.