A well - written one often has a strong sense of the original Inu x Boku SS world. The characters should be in - character, so Ririchiyo should act like she does in the anime or manga, for example. The reader insert part should be seamless, not feeling forced. It should make the reader feel like they really belong in that world.
I'm not sure about specific inu x boku ss reader insert fanfictions off the top of my head. But you can try searching on fanfiction websites like Archive of Our Own. They usually have a large collection of fanfictions for various fandoms, and you might be able to find some great inu x boku ss reader insert ones there.
The ending of Inu x Boku SS manga is bittersweet. Some relationships are solidified while others leave you wanting more. It wraps up the main storylines but leaves a few loose threads for the imagination.
One common theme could be friendship. Given the nature of the characters in 'inu x boku ss', the fanfics might explore the deepening of friendships between the characters. For example, Nobara and other characters might have storylines that revolve around how they become closer friends and support each other.
Well, it could be a story about their adventures together in the supernatural world of Inu x Boku SS. Maybe they go on a quest to solve a mystery that threatens the safety of their community. It could also explore their relationship deeper, like how they support each other during tough times.
A possible theme is sacrifice. In a dangerous situation, either Ririchiyo or Soushi might have to sacrifice something for the other. This could add depth to their relationship and create a lot of emotional moments in the fanfic. It could also show how much they truly care for each other and are willing to give up things for the sake of their relationship.
I'm sorry I can't recommend specific fanfics directly as I don't have a list of them at hand. But you can try searching on fanfic websites like Archive of Our Own. There are usually a lot of fanfics about various anime and manga series there.
One great 'Harry Potter x Reader Insert Fanfiction' is 'The Golden Trio's New Friend'. It's really engaging as it weaves the reader character into the trio's adventures in a very natural way. You get to experience Hogwarts through the eyes of the inserted character, which makes it a really immersive read.
Romance is a very popular theme. Many fanfictions explore the relationship between the reader and a favorite BNHA character. Also, the theme of fitting in. Since the reader is inserted into an already established world, they have to find their place. For example, in some fanfictions, the reader character has to prove themselves worthy to be part of a hero team. This struggle to belong is a recurring theme.