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.
Ladybug would be an obvious choice. Since they are partners, she has a vested interest in his well - being. She's also strong and capable, so she could easily take care of Chat Noir if he was injured or in trouble. Another option could be Marinette's parents. They are kind - hearted people. If Chat Noir somehow ended up at their bakery, they would surely take care of him, not knowing his true identity but seeing a young person in need.
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 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 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.
Chat Noir is often extremely helpful in fanfics. He uses his superpowers like cataclysm to protect the city and the people he cares about. For example, in many fanfics, he might save a civilian from a falling building by using his agility and acrobatic skills. His helpful nature also extends to his relationships, as he is always there to support Ladybug, whether it's in battles or in dealing with personal issues.
Well, perhaps Chat Noir got seriously injured during a fight with a powerful akumatized villain. His partner Ladybug might be worried sick as she rushes him to the hospital. Maybe his identity is at risk of being revealed there too.
Maybe Chat Noir is betrayed by his friends. This could push him to the dark side. He might start to question his values and the side he has been fighting for all along.
Well, the fanfic could explore Chat Noir's internal struggle. He already is a hero, but he wants to be more. Maybe he gets tired of being in Ladybug's shadow. So, he sets out on a journey to find his own identity as a hero. He starts by helping the citizens of Paris in small ways, like rescuing kittens from trees or helping old ladies cross the street. But then, he discovers a dark plot by an evil organization. He uses his charm, wits, and superpowers to infiltrate the group and expose their plans, proving himself as a great hero.