One great fanfiction is 'The Secrets of Gabriel Agreste'. It delves deep into his motives and the backstory that we don't often see in the show. It gives a more complex view of his character.
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.
The fanfiction should also fit well within the world of Miraculous Ladybug. It should respect the existing lore and character relationships. If it can add new elements to the story while still feeling like it belongs in the same universe, that's a great sign of a good Gabriel Agreste fanfiction.
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.
Well, a common theme is the mystery surrounding Emilie. Fanfic writers often try to solve this mystery in their own ways. For instance, they might come up with theories about where she is or what really happened to her. Love and family are also big themes. Since Emilie was part of the Agreste family, fanfictions explore the family dynamics, like how Gabriel's actions are related to his love for Emilie and how Adrien copes without his mother's presence.
A time - traveler from the future could also be an interesting main villain. This character could have seen a dark future where the Miraculouses were misused and has come back to the present to destroy them all. But in his quest, he doesn't care who gets hurt, and Emma has to stop him from changing the past and future in a bad way.
There are many interesting fanfictions out there about Adrien's car in Miraculous Ladybug. Some might focus on Adrien using his car to go on secret adventures while being Chat Noir. For example, he could use the car to quickly reach a location where an akuma attack is about to happen. The car could also be a place where he has some private moments to think about his double life and his feelings for Ladybug.
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.