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.
Perhaps in a 'Naruto Devil Trigger' fanfiction, Naruto is transported to a different dimension where Devil Trigger powers are common. He has to learn to use these powers while also finding a way back home. He could form new alliances and face new foes in this strange new world.
One interesting plot idea could be a crossover between the Devil May Cry universe and the Castlevania universe. The protagonist with Devil Trigger ability could find himself in the dark and haunted world of Castlevania. He has to use his powers to fight off the hordes of demons and undead creatures that are native to Castlevania. Maybe he teams up with a Belmont character and they exchange combat techniques and knowledge about their respective worlds.
One idea could be to have Naruto characters time - travel using the concept from Chrono Trigger. For example, Naruto could be sent back in time to the founding of Konoha and interact with the original founders. This would create a whole new set of adventures and relationships. He could learn different jutsu from that era and bring back knowledge to his own time.
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 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.
One possible plot could be that Naruto discovers a hidden power within himself that is similar to the devil trigger. This new power might make him more aggressive and powerful. He could struggle to control it at first, and his friends might be initially afraid of this new side of him. But as the story progresses, he learns to master it and use it for good, like protecting the village from a new and powerful enemy.
One idea could be Naruto discovers a hidden realm where devil hunters operate and he has to learn their ways to save his village from a devil invasion.
A new character could be introduced as the main villain. For example, a powerful demon from another realm who has the Devil Trigger ability and wants to conquer Naruto's world. This demon could have minions with similar powers and Naruto would have to figure out their weaknesses in order to defeat them.
One interesting concept could be the fusion of Naruto's ninja abilities with the supernatural elements from High School DxD. For example, imagine Naruto using his shadow clone jutsu in the middle of a devil - angel battle in the DxD universe. It would add a whole new dynamic to the battles there.
One idea could be a crossover with a magical world. The World Trigger agents suddenly find themselves in a place full of wizards and magic spells. They have to adapt their combat skills to deal with magical creatures and powerful magicians. Another plot could involve a crossover with a post - apocalyptic world. The agents must use their Trigger abilities to survive in a world filled with mutated beasts and scarce resources.
One idea could be exploring the backstories of the characters more in - depth. For example, you could write about how Mondo Owada got involved with his biker gang and what led him to his current personality. Another idea is to create an alternate ending where the killing game doesn't happen, and the students have to work together to solve a different kind of mystery, like a school - wide prank mystery. Or you could focus on a particular relationship, like the friendship between Makoto Naegi and Kyoko Kirigiri, and show how it developed before they entered Hope's Peak Academy.