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.
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 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.
If Naruto had the devil trigger in a fanfiction, it would change his character a great deal. His usual carefree and optimistic attitude could be replaced by a more cautious and sometimes even menacing one when the power takes over. He would constantly be at war with himself, between his normal self and the self that the devil trigger brings out. His relationships with his friends might also change. Some might be more wary of him, while others would try to help him through this transformation. And he would have to re - evaluate his ninja way in the face of this new and powerful force inside him.
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.
To write good character interactions, first, understand the personalities of the characters from each universe. For example, if it's a crossover between Devil May Cry and Kingdom Hearts, Dante from Devil May Cry is a cocky and confident fighter. Sora from Kingdom Hearts is more naive but determined. Their interactions could start with Dante being amused by Sora's innocence and Sora being a bit intimidated by Dante's devilish appearance. Then as they face common enemies, they start to respect each other's skills.
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 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 possible trigger could be the memory of losing Sasuke. Sasuke's departure has always been a huge emotional blow to Naruto. Every time he recalls the moment when Sasuke left the village, it might stir up intense feelings of helplessness and fear of losing his friend forever, leading to a panic attack.
The devil could be a new, mysterious character created specifically for the fanfiction. It might be an ancient evil entity that has been lurking in the shadows of the Naruto world for ages.