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 a 'Gabriel Agreste gets akumatized' fanfiction, it could start with Gabriel feeling extremely lonely and isolated. His wife is gone, and he feels like he's losing his son to a world he doesn't understand. These emotions make him vulnerable to akumatization. Once akumatized, he might have the power to freeze time in certain areas. He uses this power to set up elaborate traps for Ladybug and Chat Noir. He freezes time around their usual patrol routes and creates dangerous mazes filled with his own brand of akuma - powered obstacles. His ultimate goal is still to obtain the Miraculouses, but this time, he's more desperate and dangerous than ever.
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 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.
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.
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.
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.
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.
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.
In some 'Adrien gets akumatized fanfiction', his akumatization could be caused by a misunderstanding with his father. This could lead to his akumatized form having powers related to control or isolation, like being able to trap people in a sort of prison made of his negative emotions. The story could then focus on how Marinette, as Ladybug, tries to reach out to the real Adrien inside the akumatized form. She might have to dig deep into their relationship and use memories they've shared to break through to him.
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.
The plot could involve Adrien being akumatized after a big fight with his father. He might feel extremely alone and angry. His akumatized form could have the ability to freeze time in certain areas. Ladybug would struggle to figure out how to defeat him without hurting him too much. She might have to team up with some of the other Miraculous holders like Rena Rouge and Carapace to come up with a plan.