Perhaps a fanfiction where Gabriel and Nathalie time - travel back to when they first met. They get a chance to relive those early days and make different choices. This could lead to a completely different dynamic in their relationship and how they interact with the Miraculous and the heroes of Paris.
One idea could be that they discover a new Miraculous together. They might be on a mission to find it and in the process, their relationship deepens as they face various challenges. For example, they could encounter an enemy who is also after the same Miraculous and they have to work together to outwit them.
A different development could be that they fall out completely. Nathalie might realize that Gabriel's obsession with getting the Miraculous is too dangerous and she decides to leave his side. This could lead to her becoming an ally of the heroes and working against Gabriel, which would be a huge shift in the story.
The plot also matters. It could be a story that takes place within the existing Miraculous Ladybug universe but gives Nathalie a more central role. Maybe she has to make a difficult decision that could change the course of events in the show. Good character development for Nathalie, as well as interactions with other characters like Adrien and Marinette, can make the fanfiction great.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a large collection of Miraculous Ladybug fanfictions, including those focused on Nathalie. Another option is Fanfiction.net, which also has a wide variety of works. Just search for 'Miraculous Ladybug Nathalie' on the search bar, and you'll get a list of relevant fanfictions.
One common theme is Nathalie's loyalty to Gabriel. Many fanfictions explore her complex relationship with him, whether it's her unwavering support or the internal conflict she might feel about his more villainous actions.
One idea could be a story where the Peacock Miraculous has a unique malfunction that causes it to create unexpected akumas. For example, instead of creating the usual villains, it creates akumas that have the power to spread positive emotions but in a chaotic way that still disrupts Paris. Another idea is a fanfiction where the holder of the Peacock Miraculous has to team up with Ladybug and Chat Noir to fight against an ancient evil that is specifically drawn to the power of the Peacock. This could involve exploring the history of the Miraculous and its connection to this new threat.
There could be fanfictions where Gabriel as Hawk Moth teams up with unlikely allies. Perhaps he realizes that his way of doing things is wrong and joins forces with Ladybug and Chat Noir to fight a greater evil that threatens the entire city or even the world.
One idea could be a dance competition in Paris where Marinette and Adrien are secretly the superheroes. They use their super - agility and grace in the dance, which surprises everyone. Their opponents think they are just normal kids but they show extraordinary skills. Maybe there could be a villain who tries to disrupt the competition by using some dark magic related to the dance floor.
A crossover with the world of Sailor Moon would also be cool. The Miraculous holders could meet the Sailor Scouts. They could exchange their powers and fighting techniques. For example, Ladybug could show Sailor Moon how to use her yoyo in a more strategic way, and Sailor Moon could teach Ladybug about the power of love and friendship in her battles.
One idea could be that Hawk Moth frames Ladybug by creating false evidence of her causing chaos in Paris. For example, he uses his akumas to create illusions of Ladybug stealing or attacking civilians, which makes the public turn against her. Another idea is that a new villain frames Ladybug and Chat Noir by hacking into the media systems and broadcasting doctored footage of them doing bad things.
You could also write a fanfiction where the magic in the Miraculous Ladybug universe somehow gets intertwined with the magic in the Descendants' universe. Maybe the Miraculous themselves are affected by the dark magic from the Isle of the Lost, causing unexpected powers and challenges for Ladybug and Chat Noir as they deal with this new threat while also trying to keep their identities secret.