Perhaps the deal is made by a desperate artist who wants to become the greatest in the world. In return for his soul, the devil gives him the power to create masterpieces that can come to life. But every time he uses this power, a part of his humanity slips away, and he has to struggle with whether the price was worth it.
A deal for the devil in fanfiction could center around a detective. The detective makes a deal to solve all the world's crimes in exchange for his eternal damnation. He starts seeing the dark side of human nature more clearly as a result of the deal and has to decide if he can live with the consequences of his choice.
Perhaps in the fanfiction, Harry deals with the devil to gain knowledge about Voldemort's weaknesses. He might be at a desperate point in the fight against the Dark Lord and sees this as his only option. However, the price could be something like losing his connection to his parents' magic. This would lead to a journey of self - discovery as he tries to find new ways to fight without that power.
One idea could be Percy using his devilish powers to protect his friends in a unique way. Maybe he has the power to manipulate shadows and uses this to create illusions to mislead enemies.
Another interesting concept is to cross the devil fruits with the world of Harry Potter. Imagine a character with a devil fruit ability like the ability to turn into an animal (zoan type) trying to blend in at Hogwarts. They might have to hide their powers from the wizards while also using them to their advantage in the magical world, like using their animal form to explore hidden areas of the school.
One idea could be that Naruto is trapped in a devil - themed game world where he has to face his darkest fears in the form of his friends turned into devilish opponents. He must use his ninja skills in new and creative ways to progress through the levels.
One idea could be Naruto somehow getting a Devil Trigger power during a battle with a powerful new enemy. Maybe he discovers an ancient artifact that fuses with his chakra and gives him this new ability.
One interesting idea could be that Luffy has to rely solely on his Haki and physical training, making him even more of a badass in combat. He might develop unique fighting styles that we haven't seen in the original story where he had devil fruit powers.
One idea could be a story where Naruto discovers he has a devil gene that gives him new and dark powers. He struggles to control these powers while still maintaining his values. For example, the devil gene might make his shadow clones more menacing and powerful, but they also start to have a mind of their own.
For 'a devil is unbreakable fanfiction', you could have a plot where the devil is forced to take on a human form for a period of time. In this form, it experiences human emotions like love and compassion for the first time. This could create a conflict within the devil as it tries to reconcile these new feelings with its devilish nature. Additionally, a plot about the devil being hunted by a group of powerful sorcerers who want to use its unbreakable essence for their own dark purposes would be exciting.
One common plotline could be a character making a deal for power. For example, in some fictions, a protagonist might be desperate for wealth or status and makes a pact with the devil. The devil then sets tasks or conditions that the character has to fulfill, often leading to moral dilemmas.
Maybe an OC devil who was once a human and was turned into a devil against their will. They could struggle with their new identity while still having some human emotions, which would create complex relationships with the Hellsing characters.