In some fanfictions, Cat Noir's reaction is more complex. He is excited to know but also realizes the great responsibility that comes with this knowledge. He becomes extremely protective of Ladybug's identity. He might start to distance himself a bit in order to not accidentally give away her secret. At the same time, he has to deal with his own feelings for her which are now even more intense since he knows who she really is.
He might be shocked at first. In many fanfictions, Adrien would feel a mix of disbelief, anger, and sadness. He has always known his father as a strict but loving parent, and finding out he's Hawk Moth, a supervillain, would completely shatter his view of him.
It could be Gabriel Agreste. He has been a suspect in the original series for various reasons. He is often absent and has a lot of secrets. So in the fanfiction, it would make sense if he was finally revealed as Hawk Moth.
One interesting plotline could be that once Cat Noir discovers Ladybug's identity, he realizes they are closer in their normal lives than they thought. For example, they might be classmates or neighbors. This new knowledge makes their superhero partnership even more special and complex.
Most likely Ladybug would be involved. Since Chat Noir has a special relationship with her, any kiss she's involved in would be of great interest to him.
There are multiple consequences. Firstly, his relationship with his father will be forever changed. He'll no longer be able to trust him in the same way. Secondly, his relationship with Ladybug might be affected. She may be more cautious around him, not knowing if he'll side with his father. Thirdly, Adrien will have to face the public's reaction if the truth ever gets out. He could become a target or be shunned. All of these aspects make for a very intense and emotional storyline in fanfiction.
Well, Chat Noir finding out about the kiss in fanfiction could set off a whole chain of events. He might start to question his relationship with Ladybug. If the kiss was with someone he doesn't like, he could become overprotective or try to keep Ladybug away from that person. It could also make him more determined to win Ladybug's heart for himself.
Common themes are transformation, not just in the sense of the Miraculous transformations but also in the characters' personalities. Hawk Moth might transform from a pure - evil figure to something more complex. Another is the exploration of the Miraculous powers. Fanfics often dig deeper into how the powers work and how they can be used in different ways against Hawk Moth. And secrecy is a big theme. Keeping identities secret, Hawk Moth's secrets, and how those secrets impact the story.
Obviously, their best friends like Alya and Nino would be there. Alya could be Ladybug's maid of honor, and Nino could be Cat Noir's best man. Also, all the other superheroes they've worked with in Paris would be invited. And of course, their families, if their identities were revealed by then. Maybe even some of the citizens of Paris who have been saved by them over the years would show up to celebrate this special occasion.
Another storyline could involve a situation where they are forced to hide their feelings in public but when they are alone, like in an abandoned building that they've made their secret hideout, they engage in making - out. It adds an element of secrecy and excitement to the story.