One possibility could be that an akuma's power goes wrong and instead of attacking Chat Noir in a normal way, it turns him into a baby. Ladybug is then left to protect this baby Chat Noir while also trying to find a way to reverse the transformation. She might have to take him to Master Fu for help, all while hiding him from Hawk Moth who could potentially use this situation to his advantage.
Well, in this fanfiction, perhaps there's a magical artifact that Chat Noir accidentally activates. This causes him to regress into a baby. His kwami, Plagg, is really confused and doesn't know how to turn him back at first. Meanwhile, Ladybug has to deal with fighting crime on her own for a while until she can figure out how to get her partner back to normal. It could be a really cute and comical scenario.
A simple plot could be that there's a spell in an old book that Chat Noir reads without knowing what it does. Boom, he's a baby. Ladybug then has to babysit him while looking for a solution. She might find some clues in the Miraculous Book. It would be a story full of Ladybug's panic and Chat Noir's cute baby antics.
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.
Another possibility is that Ladybug could become the main antagonist in a sense. If Chat Noir turns dark and Ladybug tries to stop him, from Chat Noir's new perspective, Ladybug could be seen as the one standing in his way. Their relationship would be completely flipped, and Ladybug would have to find a way to bring Chat Noir back to the light while also dealing with his attacks.
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.
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.
Chat Noir could show his more protective side when it comes to the baby. His usual flirty and carefree nature might become more serious and responsible. Ladybug, on the other hand, might become more nurturing while still maintaining her strong leadership. For example, Chat Noir would be the one to always be on guard when they are out with the baby, and Ladybug would be making sure the baby is well - fed and clean.
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.
A must - read Chat Noir and Ladybug fanfiction is 'Love in the Shadows'. It has a really interesting plot where their love is tested in unexpected ways. 'The Parisian Miracles' is also great as it has a lot of action and at the same time explores the relationship between the two heroes. And 'The Secret of the Miraculous' is a fanfiction that keeps you on the edge of your seat as it reveals some secrets about their powers and how it affects their relationship.
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.
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.