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.
In such a fanfiction, it could be that Issei is bashed because he represents a different kind of hero compared to what Naruto stands for. Naruto is all about self - sacrifice and believing in others no matter what, while Issei is more about his own desires and power - seeking in a way. So when these two worlds collide in the fanfic, Issei might be made to look bad in comparison. Maybe the story has Naruto characters like Sasuke or Kakashi giving Issei a hard time because they don't approve of his behavior.
Well, often in Highschool DxD Issei bashing fanfiction, a common theme is power redistribution. Since Issei is a central figure in the original in terms of power - up and his role in battles, these fanfictions might shift the focus to other characters getting stronger or more important in different ways. It could also be about changing the overall tone of the story. Instead of Issei's more comical and pervy attitude driving parts of the plot, the fanfiction might have a more serious or different comical approach by removing or reducing his presence.
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.
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.
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.
One possible plot could be that they decide to have a traditional wedding in the Underworld. All their friends and allies from the different factions in Highschool DxD come together. Koneko might be initially hesitant but then fully supports her sister Kuroka. Issei would be nervous but excited, and there could be some comical moments as he tries to follow all the wedding customs.
Well, in such a fanfiction, Dark Naruto might use his powerful and often shadowy abilities to outwit Rias. Maybe he'd expose some of her less - than - noble actions in the Highschool DxD world. He could easily overpower her in battles, using his ninja skills and the dark powers he has. For example, his shadow clones could be used to confuse Rias and her peerage while he strikes from the shadows.
Well, it could involve Naruto characters being thrown into the Highschool DxD universe and having some sort of conflict with Rias. Maybe Naruto's skills and values clash with Rias's actions or leadership in the story. For example, Naruto might see her as too power - hungry or not treating her underlings fairly.
One popular alternate protagonist could be Kiba Yuuto. He has his own backstory and is quite popular among fans. In Issei - bashing fanfics, he could take the lead as he is strong, noble, and has his own set of values. Another could be Akeno Himejima. She has a complex personality and powerful abilities. Fans might be interested in seeing her in a more central role without Issei around.
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.