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.
One idea could be the characters from Persona 4 and Persona 5 swapping their worlds. So the Phantom Thieves end up in Inaba and have to deal with the Midnight Channel mystery. And the Investigation Team finds themselves in Tokyo and gets involved in the Metaverse. It would be really cool to see how they interact with the different environments and characters.
Chie Satonaka from Persona 4 and Ann Takamaki from Persona 5. They both have strong personalities and are very passionate about their causes. Chie with her love for kung - fu and Ann with her modelling career. They could bond over their determination and share their combat skills and life experiences with each other. It would be fun to see how they interact in a crossover scenario.
Yu Narukami and Joker would be an interesting pairing. Yu is the calm and collected leader of the Investigation Team, while Joker is the charismatic leader of the Phantom Thieves. They could exchange ideas about leading their respective groups and how to deal with the Shadows. Their different fighting styles and Persona abilities could also make for some great team - up moments in battle.
Chie and Ann would be an interesting pair. Chie is into martial arts and Ann is a model. They could have a great dynamic where Chie is in awe of Ann's fashion sense and Ann is impressed by Chie's physical strength. And they could learn from each other, like Ann teaching Chie some modeling poses and Chie teaching Ann some self - defense moves.
One interesting concept is the clash of different personas' powers. For example, when a persona from one game crosses over into another's world, their unique abilities can create unexpected story developments. Another concept is the interaction between the characters who control those personas. Their different personalities and backstories can lead to interesting dialogues and relationships.