Love and redemption are also common themes. The relationship between Sesshomaru and the OC often involves Sesshomaru learning about love through his interactions with the OC. And sometimes, the OC might have a past that needs redemption, and Sesshomaru is involved in that journey, either directly or indirectly.
Another theme is the exploration of a different side of the Inuyasha universe. The OC can bring new perspectives. Like if the OC is from a different species or culture, it allows the author to expand on the lore. For instance, an OC who is a spirit might introduce new magic or powers that interact with Sesshomaru's world in unique ways.
Another theme is the exploration of power. The OC usually has some unique power or ability, and how Sesshomaru reacts to it is a big part of the story. Sometimes they might have a power struggle, or they might learn from each other's powers.
One common theme is the power dynamic. Since Sesshomaru is a very powerful demon, often the OC has to find their place in relation to his power. It could be that the OC is initially in awe or in conflict with his power. Another theme is the exploration of the Inuyasha universe. The fanfic usually uses the setting and lore of the Inuyasha world, and the OC gets to experience it, whether it's the different demons, magic, or the feudal Japan - like setting.
Well, the theme of redemption can be quite common. The male oc may have a past he's not proud of, and Sesshomaru's presence helps him on the path to redemption. Or vice versa, Sesshomaru might find himself changing for the better. Another theme is the struggle against a common enemy. They have to put aside their differences and work together. And also, the theme of loyalty often comes up. Once they form a bond, they are loyal to each other through thick and thin.
One common theme is the exploration of their different cultures. Naruto is from the ninja world with its own set of values and traditions, while Sesshomaru is from the demon world. Their interactions often involve learning about each other's worlds.
One common theme is the idea of 'opposites attract'. Sesshomaru is often portrayed as cold and aloof, while Kagome is warm and empathetic. Their different personalities coming together is a popular concept.
One common theme is the forbidden love aspect. Since Kagome is often paired with Inuyasha in the original series, a relationship with Sesshomaru goes against the expected. Another theme is power dynamics, as Sesshomaru is a very powerful demon and Kagome has her own unique powers.
One common theme is possessiveness. Since Sesshomaru is yandere in these fanfics, his extreme possessiveness over Kagome is often a central element. He doesn't want to share her with anyone and will go to great lengths to keep her by his side.
One common theme is the journey of self - discovery for Sesshomaru. The OC's pregnancy forces him to look at himself and his place in the world in a new light. Another theme is the protection of the unborn child. Sesshomaru, being a powerful character, will go to great lengths to keep the OC and the baby safe.
One common theme is the struggle of an ordinary oc becoming worthy of a powerful being like Sesshomaru. It often involves the oc having to prove themselves, either through strength or some other special quality.
Love and redemption are also prevalent themes. Sesshomaru may be seen as a cold and aloof character at first, but through his relationship with the si oc, he may experience a change of heart. The si oc might also find redemption in their own way, perhaps by helping Sesshomaru or the people in the Inuyasha world. This theme adds a lot of depth to the stories.