One plot idea could be that Chat Noir meets a new villain who seems extremely cruel at first. But because he likes to see the good in people, he tries to have conversations with the villain during their battles. Eventually, he discovers that the villain was once a victim of a greater evil and is only lashing out. So Chat Noir helps the villain turn over a new leaf.
A plot could involve Chat Noir getting into an argument with Ladybug about his approach. Ladybug is more cautious, but Chat Noir insists on trying to redeem a group of akumatized victims who have been causing trouble repeatedly. Through his efforts, he manages to break the cycle of akumatization for those victims, showing Ladybug that his way can also be effective in the long run.
One common theme could be redemption. Since Chat Noir sees the good in everyone, characters who might be considered 'bad' in the original story could have a chance at redemption in the fanfic. For example, a minor villain might start to show signs of changing their ways because Chat Noir believes in them. Another theme could be friendship. His positive view of others might lead to the formation of unexpected friendships. He could befriend a loner or someone who is shunned by society, which would then be a central part of the story's development. Also, self - discovery for Chat Noir himself could be a theme. As he focuses on the good in others, he might also learn more about his own good qualities and how to better use them.
It often drives the story forward. For instance, when he sees good in a villain, it might lead to a redemption arc for that character, changing the whole dynamic of the conflict in the fanfic.
Perhaps Chat Noir is trapped in a different dimension where he can't use his powers easily. A kind-hearted character from that dimension decides to take care of him. This character could teach Chat Noir about their world while also trying to find a way to send him back home. Along the way, Chat Noir could learn some valuable lessons and grow as a character, all while being under the care of this new acquaintance.
One plot idea could be that Chat Noir starts to develop an eating disorder after a particularly difficult battle where he feels he didn't perform well because of his physique. His friends notice his strange eating habits but he tries to hide it. Another idea is that he gets influenced by a villain who plants the idea of the 'perfect superhero body' in his head, leading to disordered eating.
Well, in such a fanfic, there could be ideas like Chat Noir seeing alternate versions of his friends in the future. Maybe he discovers that Ladybug has a different role or identity in the future timeline.
One idea could be that they kiss during a moment of great danger, like when they are about to be defeated by a powerful villain. The kiss gives them a sudden boost of power and they are able to turn the tables. Another could be that they kiss accidentally while in civilian form, which then leads to a whole lot of confusion and self - discovery as they start to feel strange new emotions towards each other in their superhero forms.
One idea could be that Chat Noir and Ladybug discover they have a baby in the future due to some time - traveling villain's plot. They then have to figure out how to raise the baby while still protecting Paris. Another could be a magical baby is sent to them for protection, and they have to keep it a secret from Hawk Moth.
How about a fanfiction where there's a new villain in Paris who has the power to nullify the Miraculous? Ladybug and Chat Noir would have to find a way to regain their powers or find new ways to fight this powerful enemy without relying on their usual abilities.
One example could be when Chat Noir befriends a character who is initially seen as a troublemaker in the fanfic. He sees past their rough exterior and discovers their love for art, which no one else bothered to notice.
I'm not entirely sure as I haven't read this specific fanfic. But generally, if it involves Chat Noir, it might be a story that reimagines his character in a romantic or different light, perhaps with a character named 'Sweetheart' in some kind of fictional scenario.