One possible plotline could be that he's tricked by a powerful villain. Maybe the villain uses some dark magic to cloud Chat Noir's judgment. For example, they could make him believe that Ladybug has betrayed him, so in his anger and confusion, he turns evil. He might start using his powers for wrongdoings instead of protecting Paris.
It could also be that an external force, like an ancient evil entity, possesses Chat Noir. This entity takes over his mind and body, making him do evil things. His personality completely changes, and he no longer has the same moral compass. Ladybug then has to find a way to save him from this possession while still protecting Paris from his attacks.
Another plotline could be that Chat Noir discovers a dark secret about his Miraculous. Let's say he finds out that there's a hidden power within it that has a negative side - and once he taps into that, it starts to corrupt him. He then begins to act against Ladybug and the good of Paris as he's influenced by this newfound, dark power.
A common plotline is Chat Noir getting injured while trying to save civilians. He might jump in front of a dangerous attack aimed at an innocent bystander. This shows his heroic nature and self - sacrifice.
In many such fanfictions, Chat Noir might get hurt during a fight with a powerful akuma. Maybe he's outnumbered or faces a new and dangerous power. Writers often use this to create drama and show Ladybug's concern for him.
One possible fanfiction could involve Chat Noir being akumatized after a major argument with Ladybug. Maybe he feels constantly overshadowed or unappreciated. The akuma could enhance his negative feelings, making him more aggressive and less in control. His powers might become distorted, and he could start causing chaos in Paris. Ladybug would then have to find a way to reach out to the real Chat Noir inside the akumatized form and purify the akuma.
She would also be very determined to bring him back to the good side. Ladybug might search for a way to break whatever spell or influence has made Chat Noir evil. She could look for a special power - perhaps from another Miraculous or a magical item - that can reverse the change. And in the meantime, she has to be careful not to get hurt by Chat Noir while still trying to protect Paris.
One common plotline is that Ladybug has to take on a more challenging role in protecting Paris alone while Chat Noir is injured. She might struggle with the extra pressure and worry about her partner. Another could be Chat Noir dealing with his injury and trying to hide it from Ladybug so as not to worry her. For example, he could keep getting into dangerous situations while still hurt, thinking he can handle it.
Luka, who has feelings for Marinette (Ladybug), might be heartbroken at first. But he's a kind person, so he might also be a bit relieved in a way, knowing that Marinette has found someone she truly loves. The other kwamis might be all excited, chattering among themselves. Tikki might be a bit worried at first, thinking about how this relationship could change things for Ladybug, but then also be happy for her as she sees how much Ladybug cares for Chat Noir. Plagg, of course, would be making jokes and teasing Chat Noir about it non - stop.
One possible plot could be that Hawkmoth manages to finally corrupt Cat Noir. He might start causing chaos in Paris, attacking Ladybug instead of helping her. Ladybug would then be in a difficult situation, having to either try to save her partner or defeat him to protect the city.
Tikki, Ladybug's kwami, could be a central character. If Ladybug is in bed sick or injured, Tikki has to take on extra responsibilities. She could try to find a way to heal Ladybug, and in the process, we could see more of Tikki's wisdom and her relationship with Ladybug. Also, Tikki could interact with other characters in the house while Ladybug is in bed, which could lead to some humorous or heartwarming situations.
One great Miraculous Ladybug Chat Noir x OC fanfiction is 'The New Hero in Paris'. It has an interesting OC who has unique powers and a great dynamic with Chat Noir. The story is full of action and mystery.
One way is that he could use his super - senses to detect any flaws in the fanfiction's plot. Then, he can somehow communicate with the writer (maybe in a magical way) to suggest improvements. For instance, if there's a lack of action in the story, he can inspire the writer to add some of his signature acrobatic fight scenes.