A fanfiction could also explore what would happen if the Task Force from Death Note teamed up with the Phantom Thieves. They have different methods of dealing with criminals. The Task Force is more traditional law - enforcement while the Phantom Thieves target the 'hearts' of criminals. This combination could lead to some very interesting plot twists as they try to take down a common enemy.
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'.
Another interesting fanfiction could be about a character from the Phantom Thieves getting their hands on a Death Note. How would they use it? Would they be corrupted like Light? For example, Joker might use it in a more strategic way to target corrupt politicians in a different manner than Light did. He could be more focused on reforming society from the inside rather than creating a new world order.
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.
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.
Perhaps a story where the Akatsuki members from Naruto are intrigued by the power of the Death Note. They could try to obtain it for their own purposes, like achieving world domination in a different way. It would be interesting to see how characters like Itachi or Kisame would react to such a strange and powerful object that is completely different from their ninja techniques.
An interesting OC fanfiction could be about a Death Note owner who doesn't want to use it for evil but for good in a very specific way, like only targeting truly heinous criminals who keep escaping justice. They could have moral dilemmas throughout the story.
A crossover with 'Fullmetal Alchemist' could also be fascinating. Maybe Edward Elric discovers the Death Note in his search for the Philosopher's Stone. He would be conflicted because his moral code as an alchemist goes against using such a tool for killing, yet the power it offers could potentially help him achieve his goals in a different way.
Well, a possible fanfiction concept could be that Sakura enters the world of Death Note. She might meet Light Yagami and be appalled by his actions. So, she decides to use her medical knowledge (since she's a medic in Naruto) to find a way to counter the effects of the Death Note, or at least expose Light without using the unethical power of the Death Note herself. It could be a story of her moral struggle in a world with different rules of life and death.
One idea could be to create a Shinigami OC who is a bit of an outcast among the Shinigami. Maybe this OC doesn't like the idea of just writing names in the Death Note and wants to find a more 'humane' way to deal with humans, which could lead to all sorts of interesting conflicts with other Shinigami. For example, they might try to influence humans in a positive way before their time is up instead of just causing their death.
One idea could be Danny exploring the full extent of his new 'Master of Death' powers. He might discover he can not only control ghosts but also has the ability to manipulate death itself in ways he never thought possible, like bringing back certain spirits for a short time to right a wrong.
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.