In Highschool DxD OC fanfiction, the 'Power - Up' trope is quite common. Your OC starts with a certain level of power and then, through training, battles, or special encounters, they gain more power and become stronger. The 'Redemption' trope can also be used. If your OC has made mistakes in the past or has a dark history, they can strive to redeem themselves throughout the story. Also, the 'Chosen One' trope works well. Your OC could be the one chosen to save the supernatural world from a great threat, like an impending war between devils, angels, and fallen angels. This gives them a sense of purpose and importance in the story.
A great trope is the 'unexpected power - up'. Your female OC might be in a difficult situation, and suddenly, she discovers a new power within herself that helps her turn the tables. The 'protector' trope is also good. She feels the need to protect someone or something in the Highschool DxD world, like a group of innocent humans or a sacred place. And the 'identity crisis' trope can add an interesting layer to her character. She might be confused about whether she belongs to the devil side or the angel side, or something else entirely, and this internal struggle can drive the story forward.
The 'Redemption' trope is often seen in 'Highschool DxD White Dragon Emperor OC Fanfiction'. Maybe the OC has a dark past that they are trying to atone for, and their relationship with the White Dragon Emperor helps in that journey. The 'Alliance' trope is also popular, with the OC forming an alliance with the White Dragon Emperor against a common enemy. This can lead to epic battles and strategic planning within the fanfiction. And don't forget the 'Discovery' trope, where the OC discovers something new about the White Dragon Emperor or vice versa, which can change the course of the story.
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 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.
One popular trope is the 'hidden identity' trope. For example, your OC could be a powerful vampire who has been hiding their true identity from the Mikaelson family for years. Another is the 'love triangle' trope. Your OC might be caught between two main characters, like being in love with Klaus but also having a strong connection with Elijah. The 'fish out of water' trope is also great. If your OC is a new werewolf in the French Quarter, they will be out of their element dealing with the complex politics and power struggles there.
There is also 'OC's Adventure in Highschool DxD'. This story stands out because the OC is not just a bystander but actively changes the course of events in the Highschool DxD world. It has exciting plot twists and the development of the OC's powers over time is really fascinating.
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.
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 'enemies to lovers' trope. The OC and Rayla could start off on the wrong foot, perhaps having different goals or misunderstandings. But as they are forced to work together, they begin to see the good in each other. Another is the 'fish out of water' trope, where the OC comes from a different background than Rayla, and has to adapt to her world, creating humorous and heartwarming situations along the way.
One popular trope is the 'fish - out - of - water' for the OC. For example, if the OC is from a different dimension or has just discovered their powers. They'll be out of their depth in Power Girl's world at first. Another is the 'unlikely alliance' where the OC and Power Girl have to team up despite initial differences. And of course, the 'love interest' trope can also be used, but it has to be done carefully so it doesn't overshadow the main superhero aspects.