One common theme is adventure. Since both characters are from the Demon Slayer universe, they often go on demon - slaying adventures together in the fanfictions. They might face powerful demons and rely on each other's skills to survive.
Another theme is the development of their relationship. It could start from a simple acquaintance or comradeship and gradually grow into a deeper emotional bond. This can involve moments of understanding, trust - building, and even some romantic tension between female Tanjiro and Rengoku.
Tanjiro might use his sense of smell to detect hidden threats. Since his sense of smell is super - enhanced, he could sniff out any demons or traps in the way of saving Rengoku. For example, if there are demons disguised as allies, Tanjiro would be able to tell by their scent.
You can try fanfiction platforms like Archive of Our Own (AO3). There are a large number of fan-created works there. Just use the search function and type in 'female Tanjiro x Rengoku' and you'll likely find a variety of stories with different themes and writing styles.
One common theme is the exploration of their different personalities coming together. Tanjiro is very kind - hearted and Shinobu has a more complex, somewhat mysterious nature. Fanfics often show how they bridge that gap.
One possible plot twist could be that Tanjiro has to sacrifice a part of his own power to save Rengoku. Maybe he has to give up some of his newly acquired breathing techniques which he has worked so hard to master. This would show his selfless nature and his deep respect for Rengoku.
One of the main themes is adventure. Since rengoku is often involved in battles and dangerous situations, the fanfics may take the reader along on these adventures. They might face demons together and their relationship develops through these shared experiences. Also, loyalty is a big theme. Rengoku is a very loyal character, and this loyalty extends to his relationship with the reader in the fanfics.
A frequent theme is secrecy. Shinobu's yandere feelings are often something she tries to hide from others, especially from Tanjiro at first. Also, the theme of protection can be seen. Shinobu may see her yandere actions as a way to protect Tanjiro from other threats, real or imagined. This can lead to some interesting plot twists as Tanjiro may not always understand her motives.
One common theme is the exploration of gender roles. Since Tanjiro is male in the original, making him female in fanfic allows for a look at how she would be treated in the demon - slaying world as a female. Another theme is new relationships. For example, how she might interact differently with other characters like Zenitsu or Inosuke as a female.
Protection is a common theme. Tanjiro is always looking out for Nezuko, whether it's from other demons or humans who fear her. Another theme is loyalty. Nezuko's loyalty to Tanjiro is unshakable and this is often explored in fanfics.
One common theme is the growth of their relationship. It often starts from a simple acquaintance and develops into a deep emotional bond. For example, they might start as comrades in the fight against demons and gradually fall in love.
Love and friendship are very common themes. The fanfics often explore how the reader and Tanjiro develop a deep bond of love or friendship. Another common theme is adventure. Since Tanjiro is on a journey in the original story, many fanfics incorporate the reader into his adventures.