Ladybug would likely feel a sense of guilt too. She might think that she could have prevented this somehow. So, she would not only be focused on stopping Chat Noir's evil actions but also on redeeming him. She would use her intelligence and creativity, which are her strong suits, to come up with a plan to both save Chat Noir and keep Paris safe from any harm he might cause.
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 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.
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.
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.
It could still be Hawkmoth. Since he's always after the Miraculouses, he might find a way to turn Cat Noir evil and use him as a tool to finally get Ladybug's Miraculous. So Hawkmoth would be pulling the strings in the background while Cat Noir does his dirty work.
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.
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.
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.
The comic has amazing action scenes and a great development of the relationship between Ladybug and Chat Noir.
One interesting idea could be exploring different alternate universes where Ladybug and Chat Noir have different relationships. For example, in some fanfictions, they might have to deal with their identities being revealed in a more complex and emotional way compared to the original show. Another idea is the exploration of their powers in new and unique situations that the show hasn't covered yet.
First, create a well - developed OC. Give them a unique personality, backstory, and powers if applicable. For example, an OC could be a shy artist who discovers a new Miraculous. Then, focus on the relationship with Chat Noir. Build it slowly, like having them start as friends and gradually develop deeper feelings. Also, keep the tone and style similar to the original show. Use vivid descriptions of Paris and the action scenes.