Tattletale from 'Worm' could be an extremely interesting addition. Her power of hyper - awareness and her devious nature could make for some really intense interactions with the characters in 'Persona 4'. She could either be a great help in solving the Midnight Channel mystery or a major obstacle, depending on her motives.
One idea could be to have the characters from 'Worm' interact with the world of 'Persona 4'. For example, Taylor Hebert could end up in Inaba and have to deal with the Midnight Channel mystery while also trying to keep her powers in check. Maybe she teams up with the Investigation Team, and they learn from each other's combat and problem - solving skills.
Joker is definitely one of the most popular. His cool and mysterious demeanor makes him a great fit in various crossovers. Also, Morgana is often included as his sidekick. Another popular character is Makoto, with her strong - willed and intelligent personality.
Definitely Yu Narukami. He's the main character in Persona 4 and his leadership and strong Persona abilities make him a great addition to crossovers. Yosuke Hanamura is also popular. His comical nature and loyalty can add some humor and heart to the story. Chie Satonaka is another one. Her martial arts skills and determination are great traits in crossovers.
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.
You could include characters like Naruto from 'Naruto'. His ninja skills and determination would make for an interesting dynamic with the Persona 4 characters. Also, characters from 'One Piece' like Luffy. His carefree yet strong - willed nature would contrast well. And of course, characters from 'Sword Art Online' like Kirito. His sword - fighting skills and experience in virtual worlds could add a new dimension to the fanfic.
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.
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.
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.
Sasuke is a great character to include. His cool and brooding personality can create interesting dynamics in crossovers. For example, if he crosses over into the world of Attack on Titan, his skills as a ninja would be put to the test against the Titans. Another interesting character is Kakashi. His laid - back yet highly skilled nature can add humor and action to crossovers. He could be a great addition in a crossover with the world of Inuyasha, sharing his knowledge of combat and jutsu.
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.