Definitely Emilie. Fanfictions can give her a voice and a more in - depth personality. Gabriel is also quite a character in these stories. His motives and actions related to Emilie are often explored. We get to see different sides of him, like his desperation to bring her back. And then there's Adrien. His relationship with his mother, even in the absence of her physical presence, is a key aspect. His growth and how it's affected by Emilie's situation make him an engaging character in these fanfictions.
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.
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 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.
Tikki and Plagg are often featured as well. They are the Kwamis, and their interactions with their holders and with each other can be really interesting. In a camping fanfiction, Tikki might be worried about finding enough food that she can eat, and Plagg could be causing mischief by trying to steal everyone's camping food.
Lila is often a popular thief character in fanfictions. Given her manipulative nature in the original series, it's easy to imagine her taking on the role of a thief. She could be stealing things to frame Ladybug or gain more power within the school or the city.
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.
Hawk Moth is another character that shows up a lot in fanfiction. Writers like to explore his motives more deeply, or come up with scenarios where he has a change of heart or forms an unexpected alliance. His complex character makes him an interesting subject for fanfiction writers to play around with.
Aside from Ladybug and Chat Noir, Hawk Moth is quite popular in fanfiction. He is the main villain, and many fanfiction writers like to explore his backstory and motives further. There are also some side characters like Alya, Marinette's best friend, who is popular as she often gets involved in the superhero adventures in an indirect way. And of course, Adrien, who is Chat Noir, is a fan favorite due to his charming personality both as a superhero and as a civilian.
Marinette is also very popular. Fanfiction often delves into her life as a normal girl, her struggles with school, her crushes, and how she balances being Marinette with being Ladybug. Her clumsy but endearing personality gives a lot of room for interesting storylines.
Ladybug and Chat Noir are obviously very popular. They are the main characters in the original series, and in fanfiction, people love to explore their relationship further. Writers often create scenarios where they grow closer or face bigger challenges together.