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 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.
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 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 could be the crossovers with other popular game universes. For example, a crossover with the Final Fantasy series might bring in unique magic systems and character classes. Another concept could be the exploration of different personas in unexpected combinations. It allows for creative interpretations of the characters' personalities and powers.
Well, perhaps the story could start with a strange distortion in the Metaverse that links the two universes. Some of the characters from Persona 4 start having dreams about the events in Persona 5's Tokyo, and vice versa. This leads to a meeting between the two groups. They then discover that an ancient evil, long forgotten in both their histories, is trying to use the connection between the two worlds to gain enough power to rule over everything. The characters would have to learn about each other's powers and personalities quickly in order to stand a chance against this new foe.
One possible plot could be that the characters from RWBY end up in the Persona universe and have to learn how to use Personas to fight the Shadows. For example, Ruby could discover her Persona is related to her speed and scythe skills, and she uses it to quickly dispatch enemies. The team would also have to deal with the social aspects of the Persona world, like building relationships to strengthen their Personas.