In this fanfiction, it could start with Rias being in a really tough spot against Riser. Naruto might enter the scene in a very flashy way, like with a big burst of chakra. He then uses his sage mode to have an edge over Riser. In sage mode, he can sense Riser's attacks better and counter them. Naruto could also use his talk - no - jutsu to try and reason with Riser at first, but when that fails, he resorts to combat. He might use his new jutsu that he developed during his training in the Hidden Leaf Village to defeat Riser and save Rias. And after saving her, Naruto might help Rias become stronger so that she can defend herself better in the future.
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.
The main characters are Naruto, Rias, and Riser. Naruto is the savior. He comes from his own world of ninjas with his extraordinary skills and powers. His role is to rescue Rias from the clutches of Riser. Rias is the damsel in distress. She's in a difficult situation where she needs someone to save her from Riser. Riser is the antagonist. He's the one causing trouble for Rias and is the obstacle that Naruto has to overcome in order to save her.
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 plotline could be that Naruto discovers a hidden threat to Sandaime before anyone else. Maybe he stumbles upon a secret enemy's plan while on a mission. He then uses his unique skills, like his shadow clones, to outwit the enemy and save Sandaime at the last moment. For example, he could create a diversion with his clones while he sneaks up on the bad guys and disarms them.
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.
Perhaps Artemis gets lost in a strange dimension that is similar to Naruto's world. Naruto discovers her there and has to protect her from the dangerous creatures in that dimension. He uses his skills and knowledge of the area to lead her to safety. Along the way, they might form an unlikely friendship or even more, depending on the nature of the fanfiction.
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.
It could be that Daphne has a personal connection to Harry that she's not aware of until she starts investigating the strange magical disturbances around the Dursleys' place. Once she realizes it's Harry in trouble, she enlists the help of her friends or some sympathetic wizards to break Harry free. She might also face some challenges along the way, like dealing with the Dursleys' resistance or the Ministry of Magic's rules about interfering in Muggle affairs.
Perhaps Naruto is transported to Rias' world through some sort of magical or dimensional rift. He then stumbles upon Rias in a dire situation and uses his ninjutsu like the Flying Raijin to quickly reach her and get her out of harm's way. This would be an exciting start to their interaction in the fanfic.
One possible plotline is that Harry is magically transported to the Naruto world and discovers Naruto is in danger from a new, powerful enemy. Harry then uses his magic to fight alongside Naruto and his friends, saving Naruto in the process.
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.