Another theme could be the secret relationship. Given the nature of their characters in the original story, a fanfic might revolve around them having a secret relationship that they have to hide from others. This can add a lot of drama and tension to the story.
One great fanfiction is 'The Secrets of Gabriel Agreste'. It delves deep into his motives and the backstory that we don't often see in the show. It gives a more complex view of his character.
Well, a fanfic about them falling in love might also focus on the internal struggles they face. Gabriel is so focused on his goals related to the Miraculous that he might initially resist his feelings for Nathalie. Nathalie, on the other hand, might be hesitant because of her position as his assistant and the power imbalance. But as the story progresses, these barriers could be broken down, leading to a beautiful and perhaps bittersweet love story.
It could be Lila Rossi. Since she's a master manipulator, she could find out about Gabriel's identity as Hawkmoth. Maybe she tries to blackmail him into doing something for her, but Gabriel refuses. In retaliation, she somehow manages to get an akuma and send it to Gabriel, causing his akumatization. Lila might think that by akumatizing Gabriel, she can gain more power and control in Paris.
Another option is to check out Reddit communities related to the show or the pairing specifically. Subreddits like r/FanFiction or those dedicated to the show might have threads where people discuss and share gabriel x marinette fanfics.
The fanfiction should also fit well within the world of Miraculous Ladybug. It should respect the existing lore and character relationships. If it can add new elements to the story while still feeling like it belongs in the same universe, that's a great sign of a good Gabriel Agreste fanfiction.
Well, perhaps Gabriel discovers that Adrien has been secretly helping Ladybug and Chat Noir. This betrayal, in his eyes, could lead to his akumatization. As an akumatized villain, he might have the ability to create dark illusions of everyone's worst fears. He uses these illusions to try and break the will of the superheroes and force them to reveal their identities. He could also use his power to turn the citizens of Paris against Ladybug and Chat Noir, making it even more difficult for them to operate.
In the 'Chat Noir Confronts Gabriel Agreste' fanfic, Chat Noir might finally expose Gabriel as Hawkmoth. He could use his cataclysm to threaten Gabriel into revealing his true identity and his motives for being Hawkmoth. This would be a huge turning point in the story as it would end the mystery that has been building up throughout the series.
Another important factor is the exploration of the consequences of their love. How does it affect their relationships with other characters like Adrien? And how does it change the overall narrative of the Miraculous Ladybug world? A great fanfic would not just be about their love but also about how this love ripples through the entire story world, creating new story arcs and character developments.
Maybe the guide is a time - traveler. Someone from the future who has seen the consequences of Gabriel's actions and comes back to try and change the course of events. This time - traveler would know all the ins and outs of what's going to happen and could guide Gabriel towards making better choices. They could show him the destruction his actions as Hawk Moth will cause and offer him an alternative path to achieve his goals in a more positive way.