Maybe the devil loses his powers while falling in love with the mortal. This would turn the tables as he is now vulnerable and has to rely on the mortal for support. It could also lead to other supernatural beings coming after them, seeing the devil's weakened state as an opportunity.
Well, perhaps a plot twist could be that the devil is actually being controlled by an even more powerful entity, and his love for the mortal is being used as a tool to manipulate events. Or, the mortal turns out to be related to a powerful figure in the DxD universe in a way that no one expected, which completely changes the dynamics of their relationship.
The devil is surely a main character. Maybe it's a high - ranking devil with a reputation in the underworld. The mortal, on the other hand, could be a young and innocent person. There might also be side characters like the devil's friends or the mortal's family members who play important roles in the story. For example, the mortal's best friend might be suspicious of the devil and try to keep them apart.
Another possible main character could be a demon friend of the devil. This demon might be skeptical of the devil's love for the mortal and try to convince him to break it off. The interactions between these characters would shape the story and the development of the devil - mortal relationship.
Maybe Naruto could be corrupted by some dark force. He might start using his powers for evil, like attacking Konoha instead of protecting it. He could form alliances with villains from the series and use his knowledge of the village to undermine it.
A plot twist could be that the singing and dancing are actually a form of ancient ninja art that was lost. The devil knows this art and by dancing and singing with Naruto, is passing on this knowledge. However, this art has a dark side that Naruto has to overcome. He has to use his own values and the power of friendship to transform this potentially dangerous art into something positive.
Well, one possible plot twist could be that he turns out to be a long - lost relative of Bruce Wayne. Maybe he has some hidden knowledge about Wayne's family that changes the whole dynamic. For example, he could know about a secret inheritance or a past event that was covered up. Another twist could be that he's not who he seems at all, but rather an enemy in disguise, sent to infiltrate Batman's inner circle. He could start off as an ally, helping Batman with cases, but then gradually reveal his true motives which are to destroy Batman from within.
Maybe Ichigo is secretly a girl because she's on a secret mission. In the fanfiction, she could be a spy or an undercover agent. Her guise as a boy has allowed her to infiltrate certain groups or places. When her true identity as a girl is discovered, it throws the whole mission into chaos, and she has to find a way to salvage it while dealing with the personal relationships that are affected.
Well, perhaps in this 'naruto invasion of konoha fanfiction', Naruto is actually being mind - controlled to lead the invasion himself. It would be a huge shock for his friends and allies when they find out. This could lead to a lot of internal conflict as they try to figure out how to stop him without hurting him.
One possible plot twist could be that Charizard was actually trying to protect Ash by leaving. Maybe it discovered a great danger that was coming and thought that by leaving, it could draw the danger away from Ash and his other Pokémon.
A really interesting plot twist might be that Voldemort is being controlled by an even darker force, and Harry has to figure out a way to break that control while being held captive. Or perhaps Harry's friends, thinking he is dead, accidentally start a chain of events that end up helping Harry escape in a very unexpected way. For example, Hermione might cast a spell that was supposed to be for protection but ends up creating a portal right in Voldemort's lair.