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.
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.
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.
One possible plotline could be that Desiree, with her wish - granting powers, accidentally makes Danny Phantom her ward while trying to fulfill some other self - centered wish. As she raises him, Danny has to constantly struggle with the fact that Desiree's motives are often not pure. For example, she might try to use Danny to gain more power or influence in the Ghost Zone.
One possible plotline could be that Danny gets into a fight with a new and extremely powerful ghost. This ghost has some sort of unique power that can cause severe harm to Danny, like a draining ability that saps his energy and leaves him severely injured. He then has to struggle to hide his condition from his friends and family while trying to find a way to defeat the ghost.
One possible plotline could be Danny getting transported to the Naruto universe and somehow obtaining the Sharingan. He then has to learn to use it while still having his ghost powers, which could lead to some really cool battles against Naruto - universe villains.
One possible plotline could be Danny trying to hide his sickness from his enemies while still fighting ghosts. He might be weaker than usual, but his determination keeps him going.
A plotline might involve Vlad trying to isolate Danny from his friends and family in the fanfiction. By doing so, he can have more control over Danny and make him more vulnerable to his abuse. This could lead to Danny having to find ways to break free from this isolation and seek help.
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.
Well, a really cool concept can be the alternate universes in 'phantom thieves of hearts fanfiction'. The thieves could exist in a steampunk world, or a world where their actions have completely different consequences. Also, the idea of new members joining the group is quite fascinating. How would they interact with the existing thieves? What new skills or personalities would they bring?
One interesting storyline could be about the Phantom Thieves exploring a new, mysterious Palace that has different rules than the ones they've encountered before. Maybe it's a Palace where time is constantly shifting, making it difficult for them to plan their heists.