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 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.
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.
Well, start by picking a central theme that ties Unlimited Blade Works and Highschool DxD together. It could be something like power struggles or the search for identity. Next, think about how the different power systems in both will interact. In Unlimited Blade Works, there's the magic system, and in Highschool DxD, there are the various devil and angel powers. You can also play with the relationships between the characters. Maybe some of the characters from Unlimited Blade Works form unexpected alliances with the characters from Highschool DxD. And always keep in mind the tone you want to set. Is it going to be a serious, action - packed story, or a more comical take on the crossover?
One of the popular tropes in Persona fanfiction is 'alternate reality'. Writers like to explore what would happen if the events in the game took a different turn. In Highschool DxD, the 'rivalry' trope is common, especially between different devil clans. In a fanfiction that combines both, a trope could be 'culture shock'. A Persona character coming into the Highschool DxD world would experience a huge culture shock due to the different rules, species, and social structures. And also, the 'mentor - mentee' trope could be used. Maybe a powerful character from Persona becomes a mentor to a young devil in Highschool DxD, guiding them through their power - up journey.
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.
In these fanfictions, the 'loner - with - a - heart - of - gold' trope is seen a lot. The OC mercenary acts tough and independent on the outside but has a kind heart deep down. The 'rivalry' trope is another one. They might have a rivalry with another character, which could be over power, status, or a love interest. And the 'hidden - potential' trope is also popular. The mercenary initially seems like an ordinary fighter but then reveals amazing hidden powers as the story progresses.
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.
One common trope is the 'rival - turned - lover'. In Highschool DxD, there are often strong female characters who start off as rivals but then develop romantic feelings for each other in yuri fanfictions. Another trope is the 'forbidden love' aspect, considering the complex relationships and power dynamics in the original series. This adds an extra layer of tension and drama to the yuri relationships.