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.
In Highschool DxD fanfiction, a common trope regarding Issei is his growth as a character. He often starts as a pervy and weak character but gradually becomes stronger and more noble. Another aspect is his relationships, like his complex interactions with the various female characters in the series.
One popular trope is the 'Fish out of Water' scenario. For example, a character from Prototype suddenly finding themselves in the Highschool DxD world full of devils, angels, and other supernatural beings. It creates a lot of comical and interesting situations as they try to adapt.
The crossover element itself is a huge trope. Combining the unique worlds of Naruto, Highschool DxD, and the concept of ghouls allows for all kinds of interesting scenarios. Such as when the different magic systems and ninja techniques interact. For instance, a character might try to use a jutsu in the world of Highschool DxD or combine a devil's power with a ghoul's ability.
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.
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.
The 'Alternate Universe' trope is very much used. For example, an alternate universe where the events of Unlimited Blade Works and Highschool DxD are intertwined from the start. Maybe the Grail War is a regular occurrence in the Highschool DxD world. Another trope is 'Identity Reveal'. A character might be hiding their true identity from one universe while in the other, and when it's revealed, it causes a stir. Also, the 'Forbidden Love' trope can be interesting. If there are rules against a relationship between a character from Unlimited Blade Works and one from Highschool DxD, like a magical or social taboo, it can create a lot of drama and tension in the fanfiction.
One interesting Issei crossover fanfiction could be where he crosses over into the world of 'Fate/stay night'. In this scenario, Issei's devil powers might interact in unique ways with the Servants and magic in the 'Fate' universe. Another could be a crossover with 'Naruto', where Issei has to face the shinobi world with his devil abilities. There are also some crossovers where he goes to the world of 'One Piece' and has to deal with pirates and Devil Fruits while still having his own devilish powers.
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.
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.
The Highschool DxD world has a unique blend of action, comedy, and fan - service elements. Fanfiction writers can build on this foundation, adding their own twists and ideas. For example, they can create new power - up scenarios for Issei or explore different romantic pairings, which attracts a lot of readers.