A common theme is the exploration of different Persona - user groups working together. For instance, the characters from Persona 3, 4, and 5 joining forces against a greater evil. Another theme is the comparison and contrast of the different Metaverse concepts from each game. Sometimes, there's also a theme of characters from different Persona games learning from each other's experiences and growth.
One idea could be to create a fanfiction where Naruto has a secret persona as a master strategist. He hides this side of himself until a crucial battle where his strategic mind turns the tides. Another idea is to explore a fanfiction where the characters' personas are swapped. For example, Sasuke could have Naruto's positive and outgoing persona for a while, leading to some really interesting character development and interactions.
Perhaps the fanfiction focuses on the cultural clash between the two groups. The Persona 4 characters are from a small town, while the Persona 5 crew is from the big city. Their different ways of handling things and their unique Personas could lead to some interesting interactions. For example, Yu and Joker might have different leadership styles that they have to reconcile in order to succeed against the shadows.
One of the top stories in the Persona series is the journey of the protagonist in Persona 5. The story revolves around a group of high school students who become Phantom Thieves to fight against the corruption in society. They enter the Metaverse, a parallel world where people's distorted desires manifest. Another great story is from Persona 4, which has a mystery - solving aspect in a rural town where murders are occurring. And Persona 3 also has a compelling story with the theme of facing death and the Dark Hour.
First, decide on the main character or characters whose personas you want to focus on. If it's Naruto, think about what aspects of his personality you want to explore or change in the fanfiction. Then, come up with a basic plot. For example, if you want to show Naruto's darker persona, you could start with a situation where he's pushed to his limits. Next, create an outline of the story, including the beginning, middle, and end. This will help you organize your thoughts.
Another idea in the crossovers could be the exchange of technology or magic. The Velvet Room from Persona might have some connection to the magic system in Highschool DxD. Maybe the Velvet Room attendant could offer some insights or tools to the characters in Highschool DxD, like a special artifact that helps in controlling their powers better.
One great place to find Persona series crossover fanfiction is Archive of Our Own (AO3). There are many works that cross over different Persona games, like combining the characters from Persona 5 and Persona 3 in unique storylines. Some might involve the Phantom Thieves teaming up with the SEES group to face a new, more powerful enemy. Another option could be FanFiction.net, though the quality can vary more there.
Maybe the Velvet Room attendants from both games somehow get their worlds mixed up. Igor from Persona 5 and Margaret from Persona 4 have to work together to set things right. This leads to the characters from both games interacting. The Phantom Thieves might learn some new combat techniques from the Investigation Team and vice versa. They could then use these new skills to face a common threat that emerges from the merging of their two realities.
Maybe there's a strange distortion that merges parts of Inaba and Tokyo. The characters from both Persona 4 and Persona 5 find themselves in this strange new place. They start to notice that the Shadows in this merged world are more powerful and have new abilities. The Investigation Team and the Phantom Thieves must figure out how to adapt their Personas and strategies to fight these new threats while also dealing with the cultural differences between their two groups.
One great fanfiction about Persona 3 Hideotoshi could be a story that delves deeper into his backstory. Maybe it could explore his relationships with other characters in the game. For example, it could show how he interacted with his teammates during a particularly difficult mission. This would add more depth to his character and give readers a better understanding of his role in the Persona 3 universe.