One possible fanfiction could be about Rias and Sona teaming up for a school event. They might put aside their usual competitiveness and work together to organize a devil - related festival at the school. They could combine their unique skills and powers to create an amazing event that not only impresses the other students but also strengthens their bond as friends.
One way is to explore her family background more. For example, showing how her upbringing in the devil society influenced her leadership style in the student council.
A fanfiction could show how Rias and Sona first became friends. Maybe it was during a difficult time at school when they both had to face some sort of discrimination from other devils because of their different leadership styles. They found comfort in each other and decided to support one another from that moment on.
A third plot idea might be Sona trying to modernize the traditions of the devil world within the context of the school. She could face opposition from more conservative devils, and the story could explore her struggle to bring about change while still maintaining her position and respect.
Perhaps Alex Black has a mysterious past that Sona is intrigued by. They could start off as acquaintances, but as time goes on, they form a deeper bond over shared interests in magic and the supernatural world of Highschool DxD.
Power - up themes are also common. Naruto already has his powerful ninja abilities, and when combined with the magic and power systems in High School DxD, there are often scenarios where he or Sona gain new powers or find new ways to use their existing ones. For example, Naruto might learn to combine his chakra with Sona's magic for a super - attack. Another theme is friendship and alliance. Since they are from different worlds, they need to form an alliance to face common enemies. This often leads to the development of a deep friendship between them as they rely on each other's strengths.
Friendship is a common theme. Tsubaki and Sona often start with a pre - existing friendship in the fanfictions and it either deepens or is tested. For example, they might have to support each other through tough times at school or in the supernatural world.
One possibility could be that Sona's brother is a powerful figure in the underworld. He might be involved in political intrigue within the devil society. For example, he could be trying to form alliances with other noble devil families to increase the power and influence of Sona's family. Maybe he has some special abilities that are yet to be discovered by others, which he uses to his advantage in the complex power dynamics of the Highschool DxD world.
Start with small interactions. For example, have them meet in the school library while looking for books on magic. This initial encounter can be the foundation for their relationship.
Well, another plot could be that the OC is an old friend of Sona from a different dimension. They are reunited in the High School DxD world. The OC has a special ability to see the future in fragments. Using this ability, they start to foresee some dangerous events that could endanger not only Sona but the entire school. So, they work together to prevent these disasters. This could involve a lot of strategy and some cool battles against various foes.