A fanfiction where Issei and Xenovia are sent back in time to the era when the first holy swords were created. They have to blend in and figure out how to get back to their own time. During this time - traveling adventure, they meet the original wielders of the holy swords and learn valuable lessons about power, duty, and love, which would impact their relationship in the present day.
They could go on a quest to find a lost artifact. This artifact could be related to the history of the holy swords. Issei's knowledge of the underworld and Xenovia's understanding of the church could be combined to track it down. Along the way, they might face challenges from both the underworld and the church who have their own interests in the artifact.
A fanfiction idea could be that Issei and Xenovia get transported to a different dimension. In this new world, they are separated from their friends and have to find a way back. Issei tries to keep Xenovia's spirits up as they encounter various strange creatures. They discover new powers within themselves during their journey. Xenovia might also learn more about Issei's pervy side and they have some comical moments while still trying to figure out how to return home.
One interesting plot idea could be that Kuroka and Issei have to team up to solve a mystery in the Underworld. Maybe there are strange disappearances of some low - level demons, and they suspect a powerful, hidden force is behind it. As they work together, their relationship deepens from just comrades to something more romantic.
One interesting plot idea could be Issei and Yubelluna teaming up to face a powerful new enemy that neither of them can defeat alone. They have to combine their unique abilities and learn to trust each other during the battle.
A plot could involve Issei and Koneko getting trapped in an alternate dimension. In this dimension, they have to find a way back home while also dealing with the strange creatures and rules there. This forces them to rely on each other more than ever and could spark a romantic relationship as they fight for survival.
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.
One interesting plot idea could be Issei and Le Fay teaming up to face a new, powerful enemy that neither of their usual allies can handle. They could combine Issei's power of the Boosted Gear with Le Fay's magic in unique ways. Another could be a story where they accidentally end up in an alternate dimension and have to find their way back while also dealing with the strange rules and creatures there.
One idea could be a story where Issei and Rias have to face a new, powerful enemy that targets their relationship. Maybe an ancient demon who sees their bond as a threat to the traditional demon hierarchy. Another plot could be a time - travel story where they go back in time and have to deal with past events that could change their present relationship. For example, they might have to stop an event that would have otherwise led to their separation.
One idea could be that the OC has a unique power related to time manipulation. They could use this to prevent certain events that Issei was involved in, thus changing the whole dynamic of the story. For example, they might stop a particular devil attack before it starts, which would lead to different relationships forming between the various factions in the Highschool DxD universe.
One idea could be a story where Issei and Rias have to go on a special mission together in a parallel dimension. In this dimension, they lose their powers and have to rely on their wits and the bond they've developed. They face challenges that test their relationship and force them to understand each other on a deeper level.
For a fanfiction with Issei and Asia married, a great plot could be that Asia discovers a new power within herself after getting married. This power could be related to their combined love and it attracts the attention of an ancient and evil force. Issei then has to train Asia to control this power while also protecting her from the approaching danger. Or perhaps they are chosen to represent their respective groups in a tournament that has high stakes for the future of their relationship and the groups they belong to.