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.
A prevalent theme is the idea of forbidden love. Since nekos are often part of a different species or have unique characteristics, their relationship with their crush might be seen as unacceptable in the story's world. This can create a lot of tension and drama in the fanfic. Also, the theme of self - discovery can be present. As the neko pursues their crush, they might learn more about themselves and their own desires.
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.
I'm not entirely sure specifically what 'koneko neko neko visual novel' is about as it could be quite a niche or not widely known one. It might be a visual novel centered around cat - like characters (since 'neko' means cat in Japanese). Maybe it has a story about the adventures of these cat - themed characters in a fictional world.