Well, Rias Gremory herself could be a popular alternate protagonist in these fanfictions. She is a powerful devil and has a lot of influence in the original story. Without Issei, she could be the main focus, leading her peerage in different ways, and exploring her character more deeply. Also, Xenovia could be a contender. She is a strong - willed and skilled fighter, and in a story without Issei, she could have her own unique adventures and development.
In Highschool DxD Issei bashing fanfiction, Sona Sitri might be a popular alternate protagonist. She is intelligent and has her own goals and plans in the original series. Without Issei, she can shine as she manages her own peerage and deals with the political and social aspects of the devil world. Additionally, Gasper Vladi could also be an interesting choice. He has unique powers, and in a fanfic where Issei is bashed or removed, his character could be developed in new and unexpected ways.
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.
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.
Issei himself is obviously very popular as he is the central character. His growth from a perverted high school student to a powerful 'harem king' is a major draw. Rias Gremory is also highly popular. Her beauty, power, and her relationship with Issei make her a favorite in many fanfictions.
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 potential new protagonist could be Issei's other friends or comrades. Since Rias is being bashed in this type of fanfiction, they might get more screen time and development. For example, Kiba could step up and show his leadership skills and unique abilities more.
A less common but still interesting trope is the 'reversal of roles'. In some fanfictions, Issei might find himself in a world where his devil powers are seen as something different or even feared in a new way. Maybe in a world of pure angels, he is the outcast and has to prove himself not to be an evil entity. This trope allows for a lot of character exploration and development for Issei.
The popularity of High School DxD Issei fanfiction lies in the character of Issei himself. He is a complex character with a lot of potential for different storylines. His growth from an ordinary guy to a powerful being is very appealing.
Issei is a likely candidate. His over - the - top perverted nature could be a target in bashing fanfiction. Some might write stories where his actions finally catch up with him and he has to face the consequences of constantly objectifying women.
Issei could be a likely hero in such fanfiction. Even though he has a devil side, his human side might make him conflicted about the actions of some devils. He could turn against the devils who are causing harm and use his powers to bash them in order to protect his friends and the school.
Another popular plot is Murayama getting involved in Issei's battles. Since Issei is often in the middle of dangerous fights against devils and other supernatural beings, Murayama might decide to jump in and help. This could be because she has a secret ability or just because she cares deeply about Issei. In these fanfictions, their relationship grows stronger as they fight side by side.