Fanfiction often plays with different character alignments. If in this story Rias is turned into a completely evil character, far from her original 'High School DxD' self, Naruto might take it upon himself to kill her to protect the innocent. This would be a significant deviation from the source material for both characters but could make for an interesting and controversial fanfic.
One possibility is a power struggle. If the fanfiction sets up a scenario where their two worlds collide and there's a need to assert dominance, Naruto might see Rias as an obstacle and eliminate her. Another reason could be mind control. If Rias is controlled by an evil force and Naruto has no other way to stop her, he might be forced to kill her.
Power dynamics. If Rias has more power in the relationship or in the world they exist in, and she uses it in a way that makes Naruto feel small or unimportant, it could break his heart. For example, if she makes decisions about their relationship without considering his feelings because of her higher status.
The plot could involve Naruto using his knowledge of chakra to diagnose Rias' brother's illness. He might discover that it's not a normal sickness but something related to the supernatural nature of their world. For example, his chakra flow could be disrupted by a dark magic spell.
One possible plot could be that Naruto and Rias, through some magical or unexpected circumstances, find themselves in a relationship. Their pregnancy could then bring about various challenges. For example, they might face opposition from their respective worlds. Naruto's world of ninjas might not understand his relationship with Rias from a different realm, and vice versa. Their friends and allies would have different reactions, some being supportive while others being skeptical. They would also have to deal with the practical aspects of preparing for a baby, like finding a safe place to live and ensuring the child's future in two very different and complex worlds.
In a 'Naruto meets Rias' fanfiction, there could be an interesting exchange of powers. Naruto might show off his ninja skills like the Rasengan to Rias and her group. Rias, on the other hand, could introduce Naruto to the world of devils and their unique abilities. They might team up to face a common enemy, perhaps a powerful dark force that threatens both the ninja world and the devil's realm. Maybe Naruto would be a bit confused by the different rules in Rias's world, but his determination would help him adapt quickly.
Maybe it's because of the different power systems. Naruto has chakra and his ninja abilities, and Rias has her devil powers. If in the fanfiction these power systems are not properly integrated or if one overshadows the other in an unappealing way, Naruto could be depicted as hating Rias. Say, if Rias' devil powers are made to seem too overpowering and it goes against Naruto's sense of fairness in combat, it could lead to such a scenario.
Perhaps in this fanfiction, Naruto has lost his way in his own world and somehow ends up in Rias' realm. Rias, being kind - hearted, adopts him. Naruto would have to adapt to a whole new set of rules and powers. He might face challenges from other groups who are suspicious of this new addition, but with his ninja spirit, he overcomes them.
If Naruto was Rias' boyfriend in fanfiction, there would be a huge cultural clash at first. Naruto comes from a world of ninjas and honor, while Rias is in a world full of demons and magic. But as they get closer, Naruto could teach Rias about loyalty and never giving up. And Rias could introduce Naruto to the more complex social and magical aspects of her world. Their relationship could also change the power dynamics in the High School DxD world as Naruto is extremely powerful in his own right.
In this fanfiction, Naruto might introduce Rias to some of his ninja friends. And Rias could introduce Naruto to her devil comrades. This cultural exchange between the two groups could lead to some hilarious and also some tense situations. For instance, Naruto's friends might be shocked by the devil's way of life, and Rias' friends might find the ninjas' simple living strange. But through it all, Naruto and Rias would be there to smooth things over and promote understanding.
In the 'naruto rejects rias fanfiction', Naruto might reject Rias because of their different character traits. Naruto is known for his boisterous, positive, and somewhat naive nature. Rias, as a devil in her own story, has a more complex and often manipulative side. Naruto, with his straightforward ninja values, might not be able to deal with or accept Rias' way of doing things. For example, Naruto believes in hard work and loyalty among his comrades, while Rias' actions might be driven more by power plays and strategic considerations within her devil society, which Naruto could find unappealing.
In such a fanfiction, perhaps Naruto is in a new world where Rias, for some reason, completely ignores him. Maybe it could lead to Naruto feeling isolated and having to find his own way in this strange new place. He might start training alone and discover new powers out of his need to prove himself despite Rias's neglect.