Perhaps a plot twist could be that Naruto is revealed to have some sort of hidden connection to the supernatural world of Rias. Maybe he has a long - lost relative who was involved in the underworld in some way, and this connection either helps or complicates their relationship. It could also open up new story arcs and adventures for them as they explore this new aspect of Naruto's heritage together.
A plot twist might involve Rias having to choose between her duties in the Gremory clan and her love for Naruto. This could create a lot of internal conflict for her and also put a strain on their relationship. For example, she might be forced to enter an arranged marriage for the sake of her clan, but she really loves Naruto. This would lead to a complex and emotional storyline.
One idea could be that Naruto is somehow transported to the world of Rias Gremory. He, with his strong sense of justice and determination, catches Rias's eye. They start off as friends, training together. As they spend more time, they realize their feelings for each other. Naruto's unique powers also intrigue the other devils in Rias's peerage.
One idea could be that the OC is a new transfer student to the school where Rias Gremory is. They accidentally discover a hidden power within themselves that Rias is interested in. So she takes the OC under her wing, and as they train together, a deep bond forms between them.
One potential plotline could be Naruto being summoned to the world of Rias by accident. He then has to figure out how to get back home while also getting involved in the power struggles within the devil world. Maybe he becomes a key player in a battle for control of a certain territory.
In 'The Parallel Worlds' fanfiction, the plot twist is that Ichigo and Rias are actually from parallel universes that are starting to merge. They have to figure out how to stop the merge while also dealing with their feelings for each other. This creates a very unique and complex situation for them to navigate.
Well, it could be due to their different personalities. Naruto is all about hard work and loyalty to his friends in his own universe. Rias Gremory has a more complex and sometimes manipulative personality in her story. In fanfiction, the writer might want to create a conflict by making Naruto dislike her, perhaps because he sees her methods as being at odds with his own values.
In a Naruto and Rias Gremory fanfiction, Naruto's chakra system could interact in interesting ways with the magic in Rias's world. His ability to control and manipulate chakra might be seen as a form of power that the devils would either fear or try to understand. Rias, on the other hand, could introduce Naruto to the political and social aspects of her world. They could also exchange combat techniques, with Naruto showing the devils how to move with the speed and stealth of a ninja, and Rias's group teaching Naruto some devil - specific magic that could enhance his abilities in different ways.
One possible idea for this fanfiction could be Naruto being transported to the world of High School DxD. He might end up attending the same school as Rias. Given Naruto's strong sense of justice and his combat skills, he could quickly become involved in the supernatural battles and power struggles in that world. He might also form an interesting relationship with Rias, perhaps starting as friends and gradually developing into something more as they face various challenges together.
In this fanfic, an interesting concept could be the power dynamics. Harry Potter has his own set of powers and skills, and Rias Gremory has her own abilities and her peerage. When they come together, there could be a whole new power structure. Maybe they form an alliance and have to figure out how to combine their powers effectively. It could also be about the exploration of different values. Harry Potter values friendship, courage, etc., while the Rias Gremory world might have different values related to loyalty within the peerage. This contrast could lead to some great character development and storylines.
The cultural differences between Naruto's world and Rias's world could play a part. Naruto's world has a strong sense of community and loyalty within the ninja clans. Rias's world of devils has a more hierarchical and self - serving structure. These differences could cause Naruto to have a negative view of her in fanfiction.
One possible plot twist could be that their engagement is part of a political alliance between the ninja world and the supernatural world of Rias. But then they discover a hidden conspiracy within the alliance that threatens both their worlds. So they have to work together to uncover the truth while also dealing with their growing feelings for each other.