Another storyline could be a rescue mission. Rias' friends or family are kidnapped and taken to Sephiroth's world. Rias ventures there to rescue them and crosses paths with Sephiroth. At first, Sephiroth doesn't care about Rias' mission, but as she shows her determination and he sees the value in her cause, he decides to help her. In the process, they get to know each other better and their relationship evolves from indifference to mutual respect.
A potential storyline could be a battle - royale - style plot. Sephiroth and Rias are forced to participate in a tournament in a neutral dimension. They have to fight against various powerful opponents. Along the way, they learn about each other's fighting styles and motives. This could lead to an alliance between them as they realize they have a better chance of winning together. And in the end, they might face a final boss that is a threat to both their existences.
In this crossover fanfiction, perhaps Sephiroth is brought into Rias' world. Rias' peerage might initially be very wary of Sephiroth. However, his skills in combat and his knowledge of magic could prove useful. Rias could see potential in him and try to sway him from his darker path. They might go on adventures together, facing off against powerful devils and other threats, and in the process, a unique bond forms between them that is a mix of friendship and mutual admiration.
One idea could be that Sephiroth somehow gets transported to the feudal era of Inuyasha. He might initially be seen as a great threat due to his power. But as the story progresses, he could form an unlikely alliance with Inuyasha and his group against a common enemy, like a powerful demon that is even more menacing than any they've faced before.
One potential storyline could be Bucky being brought into the wizarding world by a magical accident. He then has to learn about magic from scratch. He might befriend Harry and they could train together, with Bucky teaching Harry some of his combat moves and Harry teaching Bucky magic. Another could be Bucky being on a mission in the wizarding world for some reason related to his past in the Marvel universe, and getting caught up in the fight against Voldemort.
A fanfiction could explore the relationships between Naruto, Rias, and Akeno in a more character - driven way. For example, Naruto might be intrigued by the different social structures in their world compared to his. Rias could be fascinated by Naruto's growth from an outcast to a hero. Akeno, on the other hand, might be interested in Naruto's combat techniques. They could start as strangers but gradually develop deep friendships or even more complex relationships as the story progresses.
In a Naruto - Rias' peerage fanfiction, the story might focus on how Naruto's chakra system interacts with the devil's power. For example, his ability to sense chakra could be enhanced or changed when combined with devilish senses. Also, his relationship with the members of the peerage would be interesting. He could form strong bonds with some, like Kiba in Naruto's world, he might get along well with the werewolf in Rias' peerage.
One storyline could be that an ancient evil has awakened that threatens both their worlds. This evil has the power to manipulate time and space. Wonder Woman and Captain America have to find a way to stop it. They might travel through different dimensions to gather the necessary tools or allies.
The devils that oppose Rias' group could be the antagonists. They might see Naruto as an unknown threat and try to eliminate him. Also, there could be some dark magic users who don't want Naruto interfering in their plans in Rias' world.
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.
Inuyasha would probably be very wary of Sephiroth at first. Inuyasha is used to dealing with demons and his own set of problems in his world. Sephiroth's otherworldly power and cold demeanor would make Inuyasha see him as a potential threat. He might even attack first out of instinct.
One popular sephiroth crossover fanfiction might be one where he crosses over into the Final Fantasy VII Remake universe in a different timeline. This can create interesting storylines as he interacts with the new versions of characters.