Yes, there might be. The world of fanfiction is vast and diverse. With the popularity of both Harry Potter and the concept of Ryuji Jakkan (assuming it's a known or created concept), it's very likely that some fans have combined them in their creative works. However, it may not be as mainstream as some other Harry Potter fanfictions.
A common theme is friendship - turned - love. They start as friends and gradually develop deeper romantic feelings. Another theme is mutual support. They help each other through tough times, like Haru being there for Ryuji when he has family issues or Ryuji giving Haru confidence in social situations.
One common theme is friendship. Since Ryuji is part of the B Project, fanfics often focus on his friendships with the other members. Another theme could be his journey to achieve his dreams as an idol.
One common theme could be redemption. Given Kamoshida's actions in the game, fanfic might explore his path to redemption in relation to Ryuji. Another theme could be friendship or an unlikely bond forming between them.
In 'Persona 4 Visual Novel', the gay persona might represent the diversity of the game's character creation and the exploration of different sexual orientations within the game's fictional world. It could add more depth to the relationships and storylines.
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.
A great plot could be that a new threat emerges that has the power to erase the memories of people in both Persona 4 and Persona 5's worlds. The characters from both games must work together to find the source of this threat. The Phantom Thieves could use their infiltration skills, while the Investigation Team could use their detective - like skills to gather clues. As they progress, they discover that the root of the problem lies in an ancient evil that was once sealed away but has now been awakened due to some modern - day events.
Yes. Persona 5 Royal adds new content that enriches the story. It has new characters and story arcs that blend well with the original plot. For example, the new character Kasumi brings a fresh perspective and her own story line that ties into the overall narrative in an interesting way.
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.