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.
A plot idea could be that Issei and Rias get lost in a magical realm. In this realm, their powers are suppressed, and they have to rely on their wits and the strength of their relationship to survive. They could encounter strange creatures and make new allies along the way. Or perhaps a plot where Issei has a vision of a future where Rias is in danger, and he has to figure out how to prevent it from coming true. This could involve him training extra hard and seeking out new powers to protect her.
Well, a cool plot could be that an old prophecy states that Issei and Rias' union will either bring great peace or great destruction. They then have to deal with all the political and power - hungry factions that either want to support or destroy their relationship. There could also be a plot where Issei discovers a hidden power within himself that he can only access when he and Rias are in a certain emotional state. They then have to explore and understand this new aspect of their connection to use it against their enemies.
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.
Well, a fanfiction could focus on their daily lives outside of the battles. For example, Rias and Issei could have a cooking competition with their friends from the Occult Research Club. It would be a fun and light - hearted way to show their relationship and how they interact with the other characters in a non - combat setting.
One idea could be that Issei leaves Rias because he realizes he has different goals in the supernatural world. He might want to focus on helping the weaker devils rather than being part of Rias's peerage. So he sets out on his own journey, making new allies and facing new challenges.
One interesting plot point could be the challenges they face in planning the wedding. Maybe there are rival factions who try to disrupt the preparations. Another could be the discovery of an ancient wedding ritual that they must perform, which adds a layer of mystery. Also, Issei might struggle with feeling worthy of marrying Rias, considering her noble status.
One great fanfiction idea could be that Issei discovers a new power within himself that is directly linked to Rias's heritage. Maybe he can tap into an ancient magic that only works in harmony with Rias's own powers. As they train together to master this new ability, their relationship deepens beyond just the typical master - servant bond in the Highschool DxD world.
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.
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.
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.
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.
Their relationship could develop further through shared challenges. For instance, if they face a really tough opponent together, they'll learn more about each other's strengths and weaknesses, and that can deepen their bond. Another way is through communication. If Issei starts to open up more about his insecurities and Rias does the same, they can become closer on an emotional level.