One plot idea could be Issei discovers a hidden relic related to King Arthur. This relic gives him some of Arthur's powers, leading to a power - up journey where he has to learn to control these new abilities while also facing enemies who want the relic for themselves. Another idea is that Issei somehow travels back in time to the era of King Arthur and has to interact with the legend himself, maybe even joining Arthur's knights for a while.
Well, perhaps in the fanfiction, Issei is a reincarnation of one of King Arthur's loyal knights. He starts having dreams about his past life and is drawn to all things Arthurian. This leads him to research and accidentally unlock a magical portal that transports him to a world where King Arthur is still alive. There, he has to prove his worth to Arthur and the knights by using his modern - day knowledge and his own unique skills.
Well, a plot could involve the female knight being sent on a diplomatic mission to a neighboring kingdom. Along the way, she faces various challenges and meets interesting characters. Maybe she has to negotiate with a difficult ruler while also dealing with her own feelings of being an outsider in a male - dominated world of knights. Another plot could be that she discovers a conspiracy within Camelot that threatens Arthur, and she has to work secretly to expose it.
One idea could be that Naruto has Arthur's leadership skills and uses them to unite the ninja clans in a new way. He might also inherit Arthur's sense of justice and chivalry, which would make him approach battles and conflicts differently.
One idea could be that Harry Potter discovers an ancient magic artifact that transports him to the time of King Arthur. There, he uses his magic skills to help Arthur in a battle against a powerful dark sorcerer. Maybe he teaches some of the knights a few new spells.
One way is through magic. For example, a magical spell could create a bridge between the two worlds. This would allow characters from both sides to interact. Another way is if there's a common enemy. The need to defeat this enemy would force Issei's world and King Arthur's world to work together.
One idea could be that Arthur discovers a new hidden tribe of invisibles. They have a unique power that can help save the Invisibles' world from a new threat. Another could be Arthur goes on a quest to find a magical object that was lost long ago in the Invisibles' realm, which is said to grant the finder a special wish.
One idea could be their married life facing challenges from the supernatural world. Maybe a new powerful enemy emerges that targets their relationship. Another could be a story about their attempts to start a family within the context of the supernatural realm, with all the unique difficulties that would entail.
Well, there could be a story where Issei gets injured badly, and Ravel takes care of him. During this time, they start to see each other in a new light. It could also be about them participating in a tournament together, and their teamwork and growing bond make them a force to be reckoned with.
One plot idea could be a magical adventure where Issei and Yubelluna team up to save their world from a dark force. They could discover new powers along the way and their relationship could deepen as they face challenges together.
Perhaps Issei discovers that there are exorcist families with ties to the DxD world in ways that no one suspected. He gets involved in their internal power struggles and has to choose sides while still maintaining his loyalty to his original friends. It could lead to some intense battles and moral dilemmas.
One idea could be that Issei discovers his half - vampire side gradually. At first, he notices small things like enhanced senses. He might struggle to control his newfound bloodlust, especially around his friends at school. Another plot could involve him seeking out other half - vampires or vampires to learn more about his heritage. This could lead to him getting involved in a hidden world of vampire politics.