Adrien could meet other mysterious figures in the lair. It could be an old acquaintance of his father's who has been helping with his secret plans. Or perhaps he'll come across a captive who was being held by his father for some unknown reason.
In the 'Adrien finds his father's lair' fanfiction, Adrien might meet his father's alter ego, if he has one. This would be a very intense moment as he sees a different side of his father. He could also meet some sort of supernatural being that his father has been working with. This being could either be friendly and try to help Adrien understand his father better, or be hostile and see Adrien as a threat to their plans with his father.
In this fanfiction, Adrien might discover some dark secrets about his father. Maybe he'll find out about his father's true motives or his connection to certain villains. It could also be a moment of great shock and realization for Adrien, changing his perception of his family forever.
Marinette might also play a role. Even though she doesn't have direct knowledge of Adrien's family at first, she is very observant. She could notice something odd about Adrien's behavior when he starts to search for his father's wish and offer her help. Together, they could piece together the mystery from different clues around the city.
Maybe a mysterious new character in the fanfiction. This character could be from Adrien's family's past, someone with a vendetta against his family, and decides to start by getting Adrien arrested to disrupt the family. For example, if Adrien's family wronged this character's family long ago, they see Adrien as a target to take revenge on the whole family.
Adrien would probably feel a mix of emotions. He'd likely be nervous as he approaches the prison, not knowing what to expect. Once he sees his father, there could be a sense of sadness, especially if his father looks different or broken. He might also feel anger, thinking about all the wrongs his father has committed that led to this situation.
Adrien might join his father out of curiosity. He has always had questions about his father and his actions. By joining him, he hopes to get answers to all those unanswered questions and to really know who his father is as a person.
Marinette is often a popular character. Since she has a relationship with Adrien in the original series, in the fanfiction, she could interact with the sister in various ways. Maybe they become friends, or there could be some initial jealousy from Marinette if she thinks the sister is getting too much of Adrien's attention.
Maybe he realizes how stressed his father is and wants to ease his burden.
Perhaps Adrien discovers that his father's wish was for him to take over the family business in a very traditional way. For example, in a world where they run a high - end fashion empire, his father might want Adrien to continue the legacy with the same design aesthetics and business practices.
One idea could be that Adrien initially feels shocked but also a sense of relief. He might have always had a feeling that Marinette was special, and now it all makes sense. So he starts to view her in a whole new light, with more respect and admiration for her dual identity.
Adrien's eagerness can drive the plot forward quickly. For example, he might be so impatient to meet Marinette that he makes hasty decisions, like showing up at her house unannounced. This could lead to some comical or awkward situations that add flavor to the story.