One idea could be a story about their training days before they became the wardens in Velvet Room. Maybe they had to face some tough challenges together and how they overcame those as sisters. Another could be a fanfiction where they interact with the Phantom Thieves in a more casual way outside of the Velvet Room, like going on a simple adventure in the real world.
Well, you could write a fanfiction where Justine and Caroline swap roles for a day. Caroline, who is usually more strict, has to be more gentle like Justine, and Justine, who is often the kinder one, has to be strict. This role - swap could lead to some really interesting and comical situations in the Velvet Room. Also, a story about them making a special item or tool in the Velvet Room together for the Phantom Thieves to use in their next big mission could be really engaging.
First, you should study their characters well. Watch the game cutscenes involving them, read any dialogues carefully. Then, start with a simple idea like a day in their life in the Velvet Room. For example, you could begin with Justine waking up and getting ready for the day's tasks while Caroline is already waiting impatiently.
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.
You could create an OC for your Persona OC fanfiction who is an artist. This artist OC could use their art as a way to understand and even manipulate the Metaverse. For example, they might draw scenes from the Metaverse that give clues about upcoming events or hidden locations. Their art could also be a form of therapy for other characters who are struggling with their dual lives in the real world and the Metaverse.
One idea could be creating an OC who is a transfer student with a unique past related to the Dark Hour. For example, they might have had a family member affected by Shadows long before coming to the school. Another idea is an OC with a special ability that doesn't quite fit the normal Persona - using skills that are more about mental manipulation rather than direct combat. And you could also have an OC who is initially a skeptic about the whole Persona thing but slowly gets drawn into it.
Well, you could have an OC that is a skilled hacker who gets involved with the Phantom Thieves by accident. They might have a Persona that represents a famous code - breaker from history. Also, an OC who is a foreign exchange student from a country with a very different cultural background could bring in new elements to the fanfiction. Their Persona could be influenced by their home country's folklore. Maybe an OC who is a prodigy artist and uses their art skills to create powerful items in the Metaverse, with a Persona related to a great artist from the past.
Perhaps in the fanfiction, Caroline is pregnant in a world where there are magical or supernatural elements. She could be pregnant with a special child that has unique powers, and she has to protect it from those who might want to use its power for evil.
Well, you could write a fanfiction where the Messiah in the DxD world has to adapt to the modern and often chaotic world of DxD. For instance, dealing with the different races' cultures and politics. The Messiah could start as an outsider and gradually become an important figure in uniting different groups against a powerful evil force that wants to disrupt the balance in the DxD world. This would involve a lot of character development and exploration of the DxD world's rules and hierarchies.
A really interesting plot could involve a new threat that emerges and can only be defeated by combining the powers and skills of the Persona 3 and 5 characters. For example, the combat styles of the two groups are different. The SEES members are more used to fighting in the Dark Hour, while the Phantom Thieves operate in the cognitive world. They would need to learn from each other's strategies. And on a character level, the more serious and reserved characters from Persona 3 could interact with the more flamboyant and confident characters from Persona 5, leading to some really interesting dialogue and relationship building.
Maybe there's a situation where the Velvet Room attendants from Persona 3, 4, and 5 realize that there's a threat to the collective unconscious that only the combined efforts of the three protagonists can stop. So, they somehow manage to bring the characters together. The characters then start to learn from each other's combat styles and ways of forming bonds with their friends, which helps them in their fight. For example, the protagonist of Persona 3 could teach the others about his experiences with death and how to face it head - on in battle, while the Persona 4 protagonist could share his knowledge of reading people and finding the truth behind the mysteries. The Persona 5 protagonist could contribute with his skills in stealth and infiltration.
One idea could be to create a fanfiction where Naruto has a secret persona as a master strategist. He hides this side of himself until a crucial battle where his strategic mind turns the tides. Another idea is to explore a fanfiction where the characters' personas are swapped. For example, Sasuke could have Naruto's positive and outgoing persona for a while, leading to some really interesting character development and interactions.