One interesting plot idea could be the Son of Ophis struggling to fit into the world of Highschool DxD. He might have powers that are both a blessing and a curse. For example, his powers could attract unwanted attention from various factions in the supernatural world, leading to constant battles and challenges.
A more complex plot idea could be that the Son of Ophis discovers a prophecy about himself that he is destined to either save or destroy the world of Highschool DxD. He then embarks on a journey to find out the truth behind this prophecy, all while dealing with his own internal conflicts, such as his loyalty to his mother Ophis and his newfound friends in the human and supernatural world.
One plot idea could be that Naruto is somehow transported to the world of Highschool DxD and meets Ophis. Maybe he has to help the characters in Highschool DxD deal with a threat using his ninja skills, and in the process, he forms an alliance with Ophis.
One plot idea could be Naruto being transferred to the Highschool DxD world and having to adjust to the new school environment while also dealing with the supernatural elements. He could form an unexpected friendship with Ophis, perhaps over their shared experiences of being different from the norm in their respective worlds.
One interesting plot idea could be Issei and Ophis going on a journey through different dimensions to find a way to strengthen their bond. Maybe they encounter various mythical creatures and have to overcome challenges together. Another could be a situation where they are forced to hide their relationship from the other characters in the Highschool DxD world, adding a bit of secrecy and drama.
Perhaps a plot where the Oc is on a quest to discover more about his mother, Ophis. Along the way, he forms unlikely alliances with both friends and foes, and has to navigate through various dangerous situations. For example, he could be seeking ancient artifacts related to Ophis that are scattered across different realms. This would lead him to encounter different cultures and powers within the supernatural world.
Perhaps a plotline where the characters in the harem form a sort of alliance to protect Ophis from other threats in the 'Highschool DxD' universe. They could train together and develop new powers or abilities.
The fanfiction might also focus on Ophis' powers and how they could be used in different scenarios. It could show her in situations where she has to make difficult decisions based on her abilities, adding depth to her character. Additionally, the lemon aspect, if tastefully done, can add a new layer of romance or tension to the story.
One idea could be Son Goku being transferred to the Highschool DxD world as an exchange student. He could be confused by the different rules and the presence of devils, angels and fallen angels. But his combat skills would surely make him a force to be reckoned with in the Rating Games.
One idea could be that Lucifer has a secret mission on Earth related to the events in Highschool DxD. He could be observing the main characters to see how they are handling the supernatural situations, and perhaps he has a greater plan in store for them. Maybe he's looking for a new heir or a special power that could be hidden among the students.
One possible plot could be that Godlike Naruto, with his incredible powers, enters the world of Highschool DxD. He meets Ophis and they form an unlikely alliance. Naruto's sense of justice might lead him to deal with the various power struggles in the DxD world, while Ophis, with her unique abilities, could provide support. Maybe they team up to face a common enemy that threatens both their interests.
One interesting plotline could be Issei and Ophis going on a journey to discover more about Ophis' past powers. Since Ophis is such a powerful being, exploring her origins while in a relationship with Issei can add depth. For example, they might encounter ancient beings who knew Ophis when she was first created.