The One Piece concept of the Grand Line could be translated into a sort of 'adventure path' within the Highschool DxD setting. The characters could embark on a journey through different areas of the school or the town it's in, each area having its own unique challenges and rewards, similar to the islands on the Grand Line. And just like in One Piece, they might be following a map or a prophecy that leads them to a great treasure or power.
One way is to have the Devil Fruits play a major role. Imagine a character in the Highschool DxD world who has eaten a Devil Fruit and has to hide it from others while also using its powers to gain an edge in the Rating Games or in battles against other supernatural beings. For example, a Logia - type Devil Fruit user could transform into an element to avoid attacks during a fight in the schoolyard.
You could also incorporate the One Piece idea of the Pirate Crew. In the Highschool DxD fanfiction, a group of students could form a 'Pirate Crew' - like group. They would have their own code of honor, a captain (maybe a powerful devil or an angel with a sense of justice), and they would go around the school and the surrounding areas, righting wrongs, fighting against bullies (who could be like the bad pirates in One Piece), and searching for their own version of the One Piece, which could be a powerful magical item or a way to achieve ultimate peace between the different races in their world.
One way could be through a crossover character. Imagine a character from Naruto's world ending up in the Highschool DxD universe with their Sharingan. They could use it in battles against the devils and fallen angels in the Highschool DxD world. For example, the Sharingan's ability to predict movements would give them an edge in combat.
Another unique way is to have an Incubus be the key to unlocking a hidden power within Naruto. Maybe the Incubus has a special connection to an ancient and forgotten power source in the Naruto world. When Naruto encounters the Incubus, it sets off a chain of events that leads to him accessing this new power, which is different from his normal Sage Mode or other known powers.
Their relationship could be developed uniquely by having them swap bodies for a while. This would force them to see things from each other's perspective. Issei would understand the pressures Rias faces as a devil princess, and Rias would get a taste of Issei's more carefree but determined human side.
The stray devil could be a plot device to bring out new character developments. For example, in Naruto's world, the characters are used to dealing with their own types of threats like the Akatsuki. But a stray devil has different powers and motives. Similarly, in Highschool DxD, the devils, angels, and fallen angels have their own power dynamics. The stray devil's appearance could force characters from both sides to re - evaluate their alliances and strategies.
One way to incorporate Naruto's Rinne Sharingan in a Highschool DxD fanfiction could be through a crossover event. Maybe a character from the Naruto universe accidentally ends up in the Highschool DxD world and has the Rinne Sharingan. Their unique powers could disrupt the balance in the new world, leading to various interesting plot developments. For example, they could use the Rinne Sharingan's abilities to see through illusions in the Highschool DxD world which is full of magic and deception.
In a Sharingan Naruto Highschool DxD fanfiction, the Sharingan could also play a role in terms of illusions. Imagine a scenario where the character uses Sharingan - based illusions to deceive the enemies from both worlds. They could make a Highschool DxD devil believe they are in a completely different environment like a dangerous Naruto - style forest filled with traps. And for Naruto characters, they could be made to see illusions of their worst nightmares or false threats from the Highschool DxD world, which would make for some really intense and engaging storylines.
One way is to have a Creepypasta - like curse befall Hogwarts. For instance, a curse that makes the staircases move in a truly chaotic and disorienting manner, similar to the unease that Creepypasta stories create. The students would have to navigate this new and scary situation, perhaps with some help from the teachers who are also baffled by this strange occurrence.
Their relationship could develop uniquely if Issei starts to teach Ophis about human emotions more deeply. Since Ophis is a powerful being, she may not fully understand all the nuances of love. Issei could show her through simple acts like sharing his favorite food or taking her to a place that has special meaning to him.
You can give the character a unique backstory related to the Juggernaut Drive. For instance, if they were an experiment gone wrong and that's how they got the power. This would make them stand out.
Well, fireworks can be used as a symbol. For example, in a fanfiction, every time Batman has a major victory, the people of Gotham set off fireworks. It becomes a sign of hope. Or, a new villain could create fireworks that are actually small bombs disguised as regular ones. Batman has to quickly learn to tell the difference to stop the villain's attacks.