There's a fanfic called 'Female Adrien's Adventure'. It's set in an alternate universe where Adrien is female. The story focuses on her as a Miraculous holder and how she copes with the responsibilities. It has some really cool action scenes and also some sweet moments where she interacts with her friends. It's a must - read if you like this concept.
A good fanfic in this case often has strong character development. For example, the female Adrien should have a distinct personality that still retains some of the charm of the original character. It should also have an engaging plot. Maybe she has to face different challenges because of her gender in this new version.
This event in the fanfic could also lead to new characters emerging. Maybe someone would try to take on Ladybug's mantle in a different way. Or new villains could rise, seeing the opportunity in the chaos. The magic of the Miraculous might be affected too. Since Ladybug was an important part of the magical balance, her death could have unforeseen consequences on the powers and the Miraculous themselves.
In a fanfic, Adrien might collapse injured because of some sort of curse or negative magic. For example, a new villain discovers a way to target the Miraculous holders directly. Adrien, being Cat Noir, is affected and his powers start to backfire on him, causing him to collapse. His partner, Ladybug, then has to figure out how to break the curse and save him.
A great fanfiction idea could be that Adrien and Kagami are paired up for a school project. As they work on it, they start to realize that they have more in common than they thought. They could share their dreams and fears, and this new - found closeness might make Adrien question his feelings for Ladybug a bit more.
One possible story could be that Adrien gets akumatized because he feels constantly overshadowed by Ladybug. In this fanfiction, Hawkmoth might prey on his insecurities. As an akumatized form, Adrien could have enhanced physical abilities like super strength and speed. His akumatized look might be a dark version of his Chat Noir costume, with a more menacing appearance.
One popular 'Miraculous Ladybug Adrien Harem Fanfiction' could be 'Adrien's Many Loves'. It often explores different relationships Adrien could have with multiple characters in a creative and interesting way. Another might be 'Adrien's Harem Adventures' which delves into how Adrien manages his relationships within the harem. And 'The Adrien Harem Universe' which creates a whole new world around Adrien and his potential love interests.
It could also be a case of misinformation in the fanfic. Adrien might think Ladybug has turned evil or is about to cause great harm. So, in his attempt to save the city or his loved ones, he makes the tragic decision to kill her, believing it's for the greater good.
In the fanfic, it could be related to an emotional turmoil as well. Adrien has a lot of personal issues and secrets. If something really bad happened, like he found out some truth about his family that shocked him to the core, it might have made him distracted during a fight or a dangerous situation. This distraction could have led to him being in the wrong place at the wrong time and getting caught under the rubble when something like an explosion or building collapse occurred.
Well, in the fanfic, Adrien could be in the rubble hospital due to a battle gone wrong. Maybe he got seriously injured while fighting an akuma. The hospital setting might be a place for him to have some self - reflection and also for his relationship with Ladybug to develop further, like she could visit him secretly.