First, you need to understand the characters of Issei and Raynare well. Know their personalities, motives, and backstories. Then, come up with a unique plot. It could be something like a forbidden love story between them. When writing the lemon part, if it's allowed, be tasteful. Focus on the emotions and chemistry between the characters rather than just the physical aspect. Also, make sure the story has a good flow, with a beginning, middle, and end.
One popular plot twist could be that Raynare, who initially has malicious intent towards Issei, suddenly has a change of heart due to an unexpected event. Maybe she sees Issei in a moment of great self - sacrifice and realizes there's more to him than she thought. Another plot twist might be that Issei discovers a hidden power within himself that changes their dynamic completely. This could lead to Raynare becoming more of an ally than an enemy. A third plot twist could be that an outside force tries to manipulate their relationship for its own gain, forcing them to work together against this common threat.
I'm not entirely sure specifically as it could be about a variety of things. It might be a fan - made story involving characters named Issei and Yasaka. It could be set in a fictional universe where they have an adventure together, or perhaps a story exploring their relationship in a unique way.
One common theme is redemption. Often, the fallen angel character is seeking redemption, and Issei may play a part in that process. Another theme is the power of love that can overcome differences, as they are from different species in the story's context.
Another common theme is adventure. They often embark on various quests or face challenges together in the fanfic. For example, they might be fighting against common enemies in the supernatural realm. This not only tests their abilities but also strengthens their bond.
A major element is the way their relationship affects the people around them. Their friends and allies may have different reactions to their relationship, which can create sub - plots. Moreover, the cultural and social context within their fictional world also shapes their relationship. Their different backgrounds within that world can lead to both challenges and opportunities for their relationship to grow.
There's a fanfic called 'Rias and Issei: A New Adventure'. In this one, they go on a journey together that tests their relationship. The author does a wonderful job of portraying their feelings for each other and the challenges they face as a couple in the world of Highschool DxD.
A significant element is the role - reversal scenarios. Sometimes, Rias may take a more submissive role while Issei becomes the dominant one in certain situations, and vice versa. This adds an interesting layer to their relationship. Additionally, the fanfic may explore their cultural differences, as Issei comes from a human background and Rias from a demon one. This can lead to some humorous and also some deeply emotional moments as they try to understand each other better.
Love and acceptance is also a very common theme. Given their different natures, the fanfics often explore how they come to love and accept each other for who they are. This can involve Issei accepting Ophis' more otherworldly aspects and Ophis learning to value Issei's human qualities like his compassion and determination.