One idea could be a crossover where the Phantom Thieves enter the world of Death Note. They might try to steal the Death Note from Light to stop his reign of 'justice'.
Joker from the Phantom Thieves would be really interesting. He has a mysterious and complex personality. If he were to cross over into the Death Note world, his skills in manipulating situations and people could put him in direct conflict with Light. He might be able to outwit Light in ways that others couldn't, and his sense of justice, which is more about reforming society on a personal level, would contrast greatly with Light's more extreme views.
How about a plot where the Phantom Thieves get trapped in a virtual world created by an evil hacker? In this world, their cognitive powers are limited, and they have to find a way to break free. They might meet some virtual characters who can either help or hinder them. It would be a battle of wits and skills as they try to escape and take down the hacker in the real world.
Futaba's mother, Wakaba, is an underrated character. Even though she's not alive in the main story, in fanfiction, she could be explored more. Maybe she could be brought back in some sort of virtual form in the Metaverse and have interactions with Futaba and the Phantom Thieves. It would add a whole new layer of emotion and story to the fanfiction.
One great 'Persona Phantom Thieves Fanfiction' is 'The Phantom Thieves' New Adventure'. It takes the characters on a journey through a new, mysterious Palace that has some really unique cognitive entities. It's full of action and keeps the personalities of the thieves really well - you can clearly see Joker's leadership and Panther's grace in combat.
One possible fanfiction could be about the Phantom Thieves exploring a new, mysterious palace that has unique security systems and shadow enemies. They might face challenges that test their bonds and their abilities to use their Personas in new ways.
Ryuk would probably just be amused. He is a Shinigami who has seen a lot in his existence. Fanfiction is just another strange human creation to him. He might chuckle at some of the more outlandish ideas in the fanfictions, like a character having superpowers beyond the Death Note.
Misa would likely be excited. She is a big fan of Light and the whole 'Kira' concept. In fanfiction, if there are scenarios where she has more power or a closer relationship with Light, she would be all for it. However, she might also get jealous if there are other female characters getting too close to Light in the fanfictions.
First, know the characters well. The Phantom Thieves all have distinct personalities. For example, Morgana is prideful yet also very loyal. Then, create an interesting plot. Maybe something like a new Metaverse threat that's different from what we see in the game. Also, don't forget to add some character development. Let's say, make Ann grow more confident not just in her looks but also in her combat abilities.
Joker is often a very popular character. His leadership and mysterious nature make him a central figure in many fanfictions. Then there's also Panther. Her charm and combat skills are often focused on, and her relationships with other characters are frequently explored.