One idea could be that Issei's singing is a form of therapy for himself and those around him. He sings to heal the emotional wounds of his friends who have suffered in the battles between different groups. His voice has a calming and restorative effect. And as he becomes more known for his singing, it also changes the way others view him, from just a perverted fighter to a more multi - faceted character with a unique talent.
Well, it could be about Issei using his singing as a new power or ability. Maybe his singing can charm the devils, angels, and fallen angels alike. He could perform in some sort of supernatural concert where his voice has magical effects on the audience, like making them more friendly towards each other despite their different factions.
Perhaps in this fanfiction, Issei discovers that singing is a way to communicate with a long - lost and powerful supernatural being. His songs are like keys that unlock ancient secrets or powers. He starts by singing accidentally and then realizes the power it holds. So he begins to train his voice to become stronger and use it in battles or negotiations in the world of Highschool DxD.
It could be one of the more musically inclined devils in the Highschool DxD universe. Maybe a succubus who uses music as part of her charm powers decides to teach Issei how to sing better. She might see potential in his voice and want to use him for her own purposes, like using his singing to attract more clients or gain influence in the devil society.
A simple plot could be that they compete in a school - wide tournament. Koneko's pride as a Nekoshou and Issei's determination to prove himself make for an interesting dynamic. They both face tough opponents and along the way, they offer each other advice and support, gradually becoming closer friends.
Perhaps the plot revolves around a conflict between the ideals of the Naruto world and Issei's actions in Highschool DxD. The Naruto characters, with their strong sense of justice and camaraderie, see Issei as a threat to the harmony of the new world they find themselves in. So they begin a sort of campaign against Issei. This could involve things like trying to expose his wrongdoings to the other characters in the Highschool DxD world, and at the same time, using their own powers and skills to undermine Issei's position in the school and in the supernatural hierarchy.
One idea could be that Issei is a huge Godzilla fan and he finds an ancient relic related to Godzilla. This relic starts to influence his abilities and his relationships in the Highschool DxD universe. For example, it could make him more aggressive in fights but also attract the attention of some powerful and mysterious beings who also have an interest in the relic.
He could receive a mysterious letter inviting him to a secret location on his birthday. When he arrives, he finds all the mythical creatures he has befriended over the time. They have prepared a grand feast for him with all kinds of magical foods and drinks. And there's a special performance by some of the most talented supernatural beings, like a dragon doing a fire - breathing show just for his entertainment.
Yes, there might be. There are a vast number of fanfictions out there for various combinations of popular culture elements. Since Highschool DxD has a large fan base and Godzilla is an iconic figure, it's not unlikely that some creative fans have combined the two in a fanfiction. You can try searching on popular fanfiction websites to find such works.
There could be. Fanfiction writers are known for their wild and creative crossovers. Given the popularity of Highschool DxD and the unique concept of Guyver, it's entirely possible that an Issei - Guyver fanfiction exists. You might want to check popular fanfiction websites like Archive of Our Own or FanFiction.net. Just search for relevant keywords like 'Highschool DxD Issei Guyver' and see if any relevant stories pop up.
In fanfiction, Ophis and Issei could have a mentor - mentee relationship. Ophis is extremely powerful, and she might take Issei under her wing to help him grow stronger. This could involve her teaching him about his powers and how to better control his devil abilities.
A Rias and Issei wedding fanfiction could be full of grandeur. There would be various characters from the Highschool DxD universe in attendance. The wedding might take place in a magnificent location, perhaps a grand cathedral-like place in the Underworld. Rias would look stunning in a traditional yet unique wedding dress, and Issei would be in a sharp suit. Their friends would give heartwarming speeches, and there would be a sense of celebration not just for the couple but for the unity of different factions in the story.
Yes, there are likely fanfictions out there with such a concept. In the 'Highschool DxD' universe, there are so many creative interpretations by fans. Some fanfiction writers might explore the idea of Issei taking on a Satan - like role for various reasons, perhaps to add a new twist to the story, explore different power dynamics, or to create unique character relationships.