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.
Well, he might be really surprised at first. Then he could start to see Marinette in a whole new light. Maybe he'll become more interested in her as a person, not just as a friend.
Their relationship would probably become more complex. Adrien might feel a new sense of closeness to Marinette.
Well, it could be a bit rocky at first. Adrien might be in disbelief and not know how to act around Marinette. But as time goes on, he could start to see all the times Marinette has shown the same qualities as Ladybug, like her courage and kindness. This realization would slowly bring them closer. For example, he might remember all the times Marinette has stood up to bullies at school, just like Ladybug does against villains. Eventually, they would have a deeper, more meaningful relationship.
There is this fanfic named 'Discovery of a Lifetime'. Marinette discovers that there is a parallel Miraculous world that is linked to her own. This discovery leads her on a journey to explore this new world and find out how it relates to her role as Ladybug. The story has a lot of world - building and Marinette's character development is really interesting throughout the story.
There's also 'Marinette's Hidden Talents'. In this fanfic, Adrien discovers some unique skills that Marinette has been hiding. It's a really interesting take as it builds on their characters and the bond between them. It has a lot of cute moments and a bit of mystery too.
Another amazing one is 'A Different Perspective'. This fanfiction shows the story from Adrien's point of view. It gives new insights into how he sees Marinette and how his feelings for her start to change from just friendship to something deeper.
One great plot idea could be a story where Marinette and Adrien are paired up for a school project that requires them to travel to a different city. This would force them to spend a lot of time together outside of their usual Parisian setting. They could discover new things about each other and grow closer. Another idea is a fanfiction set in a future time where they have both retired as superheroes. They start to reminisce about their past adventures and realize how much they've been through together, which reignites their feelings in a more mature way. And a third idea could be a story where they get trapped in an alternate dimension. There, they have to rely solely on each other to find a way back home, which deepens their bond.
He could be shocked at first. I mean, he has been so close to Marinette all this time without realizing she was Ladybug. His reaction might include disbelief and then a quick acceptance as he starts to piece things together in his mind.
One interesting plot could be Adrian's initial shock and then his realization of all the times Marinette's behavior as Ladybug made sense in her civilian form. For example, her courage as Ladybug might be seen as her standing up to Chloe in her Marinette guise.
A good fanfiction in this regard should first have a believable character development. For example, showing how Marinette overcomes her shyness around Adrien gradually. Second, it should have interesting plot twists. Maybe something that makes their relationship take an unexpected turn. For instance, a new power or a new character that affects their dynamic.