A great storyline might involve a fusion of the two worlds' mechanics. For example, the Velvet Room from Persona could have some connection to Joker's world. The Igor-like figure could offer Joker and his friends some new kind of power or knowledge that they use to combat a common foe. Or perhaps the Persona characters learn about Joker's Metaverse Navigator and try to replicate it in their world, which leads to all sorts of unforeseen consequences and adventures.
Some characters might not get along so well. For instance, a more serious Persona character might find Joker's carefree attitude a bit off - putting. However, as they face challenges together, they would start to understand and respect each other. The Persona characters could introduce Joker and his friends to the concept of the Collective Unconscious in more detail, and the Phantom Thieves could show them how to use their skills in a more urban, modern - day setting. There would be a lot of banter and friendly competition between the two groups as they learn from each other.
The 'joker and his girls persona 5 fanfiction' could potentially focus on various storylines. It might involve adventures that Joker and the girls embark on together. They could be facing off against powerful enemies in the Persona 5 universe. Another possibility is that it explores the emotional bonds between them. How they support each other through difficult times, and how their relationships evolve over the course of the story.
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 idea could be that Kirby swallows a Persona and gains its powers. This would lead to some really interesting battles and interactions. Kirby could then use the Persona's skills in the Dream Land setting, creating a whole new dynamic.
I really like Yu Narukami from Persona 4 in the crossover. He brings his calm and collected leadership from his own game. His ability to analyze situations and keep the group together is really cool when he's interacting with the Persona 5 characters.
A great moment is when they combine their Personas' powers. Imagine the unique abilities of Persona 4's personas merging with those of Persona 5 in a battle against a common enemy. It's like a display of a whole new level of combat strategy that makes the crossover fanfiction exciting.
Another possible theme is growth. The girls, with Joker's influence, could be growing as characters. They might be learning new skills, becoming more confident, or maturing in their understanding of the world around them. Also, Joker himself might be growing through his interactions with them.
One popular theme is the female Joker's journey of self - discovery within the Metaverse. Another is her relationships with the other Phantom Thieves, like how she bonds with them and overcomes challenges together. Also, stories might focus on her unique skills and how she uses them in battles against Shadows.