One idea could be exploring the backstory of the devil's guardian. Maybe they were once a normal being who made a deal with a powerful entity to become a guardian. This could involve their past life, family, and the events that led them to this role.
One idea could be a fanfic where a new character is a guardian of Naruto's hidden powers. This character secretly protects Naruto from those who might try to steal or misuse his power.
For a good 'A Devil's Guardian' fanfic, the world-building has to be immersive and consistent with the source material. Also, the writing style should be compelling and capture the tone of the original work. Good character interactions and unexpected twists can make it even better.
One idea could be a story where Naruto somehow gets a devil fruit that gives him the power to control water. He could use it in his battles in the Naruto world, like creating huge water jutsus with the unique properties of the devil fruit power. Another idea is to have a crossover where characters from One Piece come to the Naruto world and Naruto has to deal with the chaos that the devil fruits bring along.
One idea could be exploring Dante's early days as a devil hunter, like his first encounter with a really powerful demon. It would show his growth and how he developed his skills.
The fanfic usually involves Zack Fair taking on the role of a protector against evil forces, with lots of action and adventure.
One idea could be to create an OC who is a descendant of a powerful demon hunter family. This OC could have a unique weapon that combines the abilities of different Devil May Cry weapons, like a sword that can also fire bullets. Another idea is an OC with a special power to control time briefly during battles, adding a new dynamic to the fight scenes. You could also have an OC who is a half - demon, half - angel, struggling with their dual nature and how it affects their place in the Devil May Cry universe.
One idea could be that an angel and a devil are forced to work together to save the world from a greater evil. They start off hating each other but gradually learn to respect and even care for one another.
One idea could be a character with a devil bloodline being initially shunned in the Hidden Leaf Village but then proving their worth in a major battle. For example, they could use their unique devil - like abilities, such as enhanced speed and strength, to turn the tide against a powerful enemy.
One idea could be that your OC is a guardian of a long - lost magical artifact from Fairy Tail's history. They have to protect it from a dark guild that wants to use its power for evil. Maybe they team up with some of the main Fairy Tail characters along the way, like Natsu and Lucy, and there are epic battles to keep the artifact safe.
One idea could be Percy being reluctantly recruited by the Hunt. He has to learn their ways while still holding onto his own identity. Maybe he discovers a new power related to the Hunt that no one expected.