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.
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.
One possible plot could be that Gabriel Agreste, in his search for a way to achieve his goals (maybe related to his fashion empire or his secret identity as Hawk Moth), gets help from an unexpected guide. This guide might be a character who has unique knowledge of the Miraculous or the hidden aspects of Paris. For example, the guide could be an old acquaintance who has connections to the magic that surrounds the Miraculous. They might lead Gabriel to discover new powers or secrets that could change his approach to his plans.
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.
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.
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, 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.
One possible plot could be that Ladybug is betrayed by someone she deeply trusts, like Chat Noir accidentally revealing her identity to Hawkmoth. This shatters her world and makes her vulnerable to Hawkmoth's akuma. As Ladybug gets akumatized, Paris is in chaos as she starts using her powers in a negative way. The other heroes, like Rena Rouge and Carapace, try to find a way to save her and break the akuma's hold.
Hawkmoth is usually the main antagonist. Since he is the one who akumatizes Marinette in the first place. He takes advantage of her negative emotions, like her insecurities or anger, to turn her into an akuma and use her for his own evil plans.
Well, if Plagg gets akumatized in fanfiction, it could lead to some really chaotic situations. Plagg's powers are already quite strong as a kwami, and with the akuma's influence, they might become uncontrollable. For example, his cataclysm power could be misused on a large scale, causing destruction all over Paris. Maybe he would turn against Adrien and the other heroes, creating a big rift in their relationships and making it extremely difficult for them to fight back.