It could be that an ancient curse is placed on Keiko because of Yusuke's actions in the spirit world. The curse slowly drains her life force. Yusuke tries everything he can to break the curse, but fails. When Keiko dies, he is filled with guilt. He then embarks on a quest to find the originator of the curse. Along the way, he uncovers more about his own past and the hidden powers within him that he may be able to use to either bring Keiko back or at least make sure such a tragedy never happens again.
Perhaps in this fanfiction, Keiko discovers a dark secret about the spirit world that some powerful entities don't want her to know. They decide to silence her, leading to her death. Yusuke then has to fight against not only his grief but also the system that allowed this to happen. He might team up with some unlikely allies, like other outcast spirits, to expose the truth and also try to find a way to revive Keiko, even if it means defying the laws of the spirit world.
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.
Yusuke would likely become more withdrawn. He was always a bit of a rough - around - the - edges character, but Keiko's death would make him more somber. He might lose his sense of humor that he sometimes showed around her.
Growth is another theme. Yusuke often goes through a lot of changes, and Keiko is there. Fanfictions might show how they both grow as individuals and as a couple in relation to those changes. For example, Yusuke becoming more responsible and Keiko being more understanding.
Yusuke might cope by throwing himself into his work as a spirit detective. He could try to distract himself from the pain of losing Keiko by taking on more and more dangerous cases. This would be his way of running away from his emotions at first.
One way the story could develop is that Yusuke goes into a deep state of grief and denial at first. He might start to isolate himself from his friends and the spirit world as well. He could be haunted by memories of Keiko and constantly wonder if there was something he could have done to prevent her death.
One idea could be that Demon Yusuke has to protect Keiko from other malevolent demons while hiding his true nature from her at first. As the story progresses, Keiko discovers his secret and has to come to terms with it. Another plot could be a time - travel scenario where Yusuke goes back in time to save Keiko from a danger that she faced in the past, which changes the course of their relationship in the present.
Yes, there are quite a few. You can start by looking on fanfiction platforms like Archive of Our Own. Just search for 'yusuke and keiko love' and you'll find a variety of stories written by different authors with different takes on their relationship.
One plot idea could be that Demon Yusuke has to protect Keiko from other demons who see her as a way to get to Yusuke. Another could be Yusuke trying to balance his demon side with his relationship with Keiko, like when he has to hide his new powers and instincts from her at first. Maybe there could also be a plot where Keiko discovers a way to temporarily become a demon herself to be closer to Yusuke's world.
There's this fanfic called 'Together Always' which is really interesting. It delves into how Yusuke and Keiko handle the challenges in their relationship. It shows Yusuke's growth as a character in the context of his love for Keiko. Keiko, on the other hand, is depicted as being very supportive of Yusuke's journey.
You can try fanfiction.net. It has a large collection of various fanfictions, including those of Yusuke and Keiko.
There's also 'The Unspoken Bond'. This fanfiction delves into the unsaid feelings between Yusuke and Keiko. It has a really nice build - up of their emotional connection through various events.