His friends like Kuwabara and Kurama might also join in the rescue. Kuwabara with his tough fighting spirit and Kurama with his cunning and powerful abilities could be a great help in getting Keiko back from Toguro.
In such a fanfiction, perhaps Toguro kidnaps Keiko to use her as bait to lure Yusuke. He might think that Yusuke will come to rescue her no matter what, and then he can have a rematch with Yusuke on his own terms.
Well, perhaps in this fanfiction, Sakyo and Toguro kidnap Keiko because they think she has some special power or knowledge. Maybe they believe she can lead them to a powerful artifact. Keiko, being a strong - willed character, tries to resist them and find a way to escape. Meanwhile, her friends are desperately searching for her, and they end up having to team up with some unexpected allies to take on Sakyo and Toguro.
Another aspect could be a fanfiction set in an alternate universe where Genkai and Younger Toguro are allies against a greater evil. They could use their combined skills and knowledge to fight off powerful demons. This would show a different side of their relationship, more of a partnership than what we might expect from their normal interactions in the original series.
One idea could be a crossover with a popular shonen series like Naruto. Toguro could be brought into the Naruto world as a powerful rogue ninja. His immense strength and dark nature would make him a formidable adversary for the Konoha ninjas. Another idea is a crossover with One Piece, where Toguro joins a pirate crew as a fearsome fighter, perhaps with his own hidden motives for being on the Grand Line.
In some fanfiction, they could be involved in a tournament together. Genkai as a mentor figure guiding Younger Toguro through the challenges of the tournament, teaching him strategies and how to control his emotions during the battles. This would show their relationship evolving in a high - stress environment.
One possible character could be Keiko's father. He is a powerful and influential man in their community. When he finds out about Keiko's kidnapping, he uses all of his resources, including his connections with the local police and his own private security force, to try and rescue her. Then there could be a detective, Inspector Kenji, who is assigned to the case. He has a reputation for being tenacious and smart, and he starts to piece together the clues about Rando's identity and motives.
One possible plot could be that Keiko is caught in the crossfire of a demon battle that Yusuke is involved in. Yusuke, being a spirit detective, has many enemies. Maybe a powerful demon targets Keiko to get back at Yusuke. After her death, Yusuke could go into a deep state of despair and vow revenge. He might journey to the spirit world to find a way to bring her back, facing numerous challenges along the way.
In a crossover with the Fate series, Saber could be paired with Toguro. Saber's sense of honor and her noble fighting style would clash with Toguro's brute force and underhanded tactics. It would create a really interesting dynamic, whether they are on opposing sides or forced to work together for some greater purpose.