The antagonists in the story would likely be either pleased or suspicious. If they were the ones who caused the disappearance, they would be pleased at first. But if they didn't, they would be suspicious that the thieves are up to something. They might increase their security or start looking for the thieves themselves, fearing that the thieves are planning a big comeback or a counter - attack.
The phantom thieves might disappear because of a false accusation. A powerful figure frames them for a crime they didn't commit. To clear their names and also protect themselves from the immediate danger of being arrested, they disappear. They then operate in the shadows, trying to gather evidence to prove their innocence. Along the way, they encounter new allies and enemies who are either helping or hindering their quest to return to their normal lives.
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.
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.
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.
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.
At first, the village would be thrown into chaos. People would be speculating all kinds of reasons for his disappearance. His closest friends would be heartbroken and determined to find him. The elders of the village would be concerned about the political implications. Without Naruto, their main source of power and protection seems to be gone. However, some might also see it as an opportunity for a new era in the village, one that is not so reliant on a single individual, but this view would be in the minority at the start.
Joker is often very popular in fanfiction. As the leader of the Phantom Thieves, he has a lot of mystery and charisma. Writers like to explore his backstory further or put him in different, challenging situations where he has to use his wits and Persona powers in new ways.
Well, it depends on the nature of the fanfiction. Some anime characters might be amused if the fanfiction is light - hearted and full of comical situations. For example, if it's a fanfic about a cooking competition among the characters from a shonen anime, they might react with good - natured laughter and competitiveness. They could also be confused if the fanfiction takes their story in a very strange or unexpected direction, like suddenly making a character from a historical anime end up in a space adventure.
Some characters may be excited. They might enjoy seeing their world expanded in new ways. For example, a side character could be happy to see themselves getting more spotlight in 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.