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.
Well, some popular rengoku x reader fanfics can be found on Wattpad as well. Look for ones with high ratings and good reviews. They usually offer different storylines, from adventure - filled ones where rengoku and the reader go on quests together, to more romantic and heart - warming stories about their relationship.
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.
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 idea could be a rengoku fanfic where he trains a new group of young slayers. It could focus on his unique teaching methods and how he instills his values in them.
Tanjiro would be a great character to pair with rengoku. Their different personalities could create an interesting interaction. Tanjiro's kind - hearted nature and rengoku's fiery determination could lead to some great storylines.