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.
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.
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.
There's a fanfiction called 'The Miraculous Origins'. It delves deeper into the backstory of the Miraculous. It explores how the Miraculous were created and how they came to be in the hands of the current holders. It's a long read but worth it for fans who want to know more about the lore of the Miraculous Ladybug Cat Noir universe.
Definitely Tikki and Plagg. These Kwamis are essential to the Miraculous universe. Their interactions with Cat Noir and Ladybug, as well as their own little quirks and personalities, can make for really fun fanfiction. Additionally, Sabrina, Chloe's sidekick, can be an interesting character to include. She could have her own story of breaking free from Chloe's control or getting involved in the hero - villain drama in some unexpected way.
Marinette and Adrien, as Ladybug and Cat Noir, finally decided to tie the knot. The city of Paris was filled with excitement. The wedding took place in a grand cathedral. Ladybug walked down the aisle in a beautiful white dress with red accents, while Cat Noir waited at the altar in his dashing black suit. Their kwamis were there too, Tikki and Plagg, looking on with pride. After the ceremony, they transformed briefly to soar over Paris as a newlywed superhero couple, sharing a kiss in the sky.
To write a good Miraculous Ladybug Cat Noir fanfiction, you should start with a strong concept. It could be a 'what - if' scenario, like what if Ladybug and Cat Noir's identities were revealed early. Then, build your story around that. Also, pay attention to the relationships between the characters. The friendship and budding romance between Ladybug and Cat Noir are important elements. Don't forget to add some twists and turns to keep the readers interested. Maybe a character they thought was a friend turns out to be working for the villain.
Well, a common fanfiction idea is to give the characters new powers or challenges. Maybe Ladybug has to deal with a new type of akuma that nullifies her usual powers. Or Cat Noir has to face an enemy alone without Ladybug's help. Also, there are fanfictions that play with the idea of the characters' identities being revealed in unexpected ways, which then leads to a whole new set of problems and solutions for them.
In a fanfiction, Alix might get the Bunny Miraculous in a rather unexpected way. For instance, the Miraculous could be sent to her from an alternate future where she is the only hope. Once she has it, she struggles at first to understand the time - jumping abilities. But as she practices, she becomes a crucial asset in the fight against Hawk Moth. She could use the power to go back and prevent some of Hawk Moth's early akumatizations, changing the course of the battle.
In some ways, it's a very interesting plot twist. It allows for exploration of different character aspects. Cat Noir stealing the Miraculous can show his more complex side, his insecurities or the lengths he might go to in certain situations. However, it also risks ruining the positive relationship that fans love between him and Ladybug if not handled carefully.