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.
Another theme is sacrifice. Yusuke has sacrificed a lot in his life, and Keiko is often by his side. Fanfics may show how they both make sacrifices for each other and for their relationship. For example, Yusuke might sacrifice his time or safety to protect Keiko, and she might sacrifice her own desires for Yusuke's well - being.
One common theme is the exploration of their hidden desires. Since the 'lemon' aspect often implies a more sensual side, it could be about Yusuke and Keiko exploring their physical attraction that they may have suppressed in the original story. Another theme could be the intensification of their emotional bond. Through various situations in the fanfiction, their love and understanding for each other can grow deeper.
Yusuke would likely become more withdrawn.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.
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 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.
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.
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.
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.
You can try fanfiction.net. It has a large collection of various fanfictions, including those of Yusuke and Keiko.
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.