In some 'Naruto Rias sick' fanfiction, it could be that Rias is cursed by an enemy. Naruto, being the hero he is, feels responsible for helping her. He uses his knowledge of chakra and seals to try and break the curse. Meanwhile, Rias is weak from the curse and her devil powers are waning. As Naruto delves deeper into finding a solution, he uncovers secrets about both worlds that are related to the curse. This could lead to a series of adventures where they have to overcome various obstacles together, all while Rias is struggling with the effects of the sickness.
One possible fanfiction idea could be that Rias' sick brother is a key factor in a power struggle within their supernatural world. Naruto, being an outsider with his own set of skills and values, could get involved. Maybe he forms an alliance with Rias to protect her brother from those who would use his illness to their advantage.
One possible plotline could be that Naruto, with his powerful ninja skills, suddenly appears in the world where Rias is in trouble with Riser. He might use his shadow clone jutsu to distract Riser while he rescues Rias. Another could be that Naruto uses his rasengan to break through any barriers or attacks that Riser has set up to keep Rias captive. Maybe Naruto is summoned there by some sort of magic or a wish that Rias unknowingly made, and he then uses his chakra to overpower Riser and save Rias.
One possible plotline could be that Naruto is transported to the Highschool DxD world and immediately notices Rias's overbearing attitude. He decides to oppose her and starts to gather allies among the other students and characters who are also fed up with her ways. As a result, a power struggle ensues between Naruto and Rias.
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 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.
One possible plotline could be that Naruto has his own mission or purpose that is in conflict with Rias's plans or the world she is a part of. So, he decides to leave to pursue his own goals. For example, he might need to go back to the Hidden Leaf to protect it from a new threat that only he can handle. Another could be that Naruto discovers some dark secret about Rias or her group and can't stay with them any longer. He values his own moral code too much to be associated with something he deems wrong.
Maybe they meet when they are very young. Naruto, being an outcast in his village, and Rias, who might be facing some politics in her devil world. They bond over their shared feelings of loneliness and not being fully understood. As they grow up, they help each other out. For example, when Naruto has to face powerful enemies in the Chunin Exams, Rias could use her devil powers secretly to assist him without getting caught.
Well, it might be that Rias sees Naruto as a rival in terms of power or influence in a new setting. So, she uses her devilish powers to abuse Naruto. In the story, Naruto could start out confused and hurt but then draw on his ninja training and positive relationships back home to slowly turn the tables and make Rias see the error of her ways. Maybe he even teaches her about friendship and loyalty, which are concepts not as prominent in her world.
A frequent plotline is the training aspect. Naruto could teach some of Rias's peerage his ninja training techniques, which would be quite different from the normal training in the devil world. This can lead to new power - ups for the characters. There are also plotlines where Naruto is mistaken for a powerful devil or some other supernatural being in the 'High School DxD' world. This misunderstanding can create a lot of chaos and interesting story developments as he tries to clear his name while also dealing with Rias's harem and the various factions in that world.
Another possible reason is that Naruto might be protective of his own story's integrity. He has been through a lot in his own series, and the Rias fanfiction might seem like an intrusion or a misappropriation of the idea of fictional characters. It's like his world and story are his own, and the fanfiction is trying to merge it with something else in a way that he doesn't like. He may feel that his own story should be kept separate from this other fan - created content.