If Adrien and Marinette were akumatized, their relationship would become very strained. As akumatized individuals, they would be full of negative emotions which could cloud their judgment. Adrien, who usually has a soft spot for Marinette, might not recognize her in his akumatized state and could even attack her. Marinette, too, would be in a different mental state and might see Adrien as an enemy, forgetting their previous friendship and any budding romantic feelings.
If they were akumatized, their relationship would be completely transformed. Normally, there's a lot of unspoken affection between them. But in their akumatized forms, they would be in a state of emotional upheaval. Adrien might not show his usual charm and kindness towards Marinette, and instead be cold or aggressive. Marinette, who is usually strong - willed and kind, would become a more menacing figure towards Adrien. It would take a great deal of effort and understanding to repair their relationship once they are back to normal, as they would have to face the fact that they were capable of hurting each other while akumatized.
His relationship with Ladybug would be severely strained. Since he's akumatized, Ladybug would see him as an enemy she has to defeat, which would be a huge shift from their usual partnership. With his father, Gabriel might initially be shocked but then could see it as an opportunity to control Adrien's new powers for his own evil plans. And his friends like Nino would be very confused and scared, not understanding why Adrien has turned into this akumatized being and would try to find a way to bring him back to normal.
A possible plot is that Adrien is akumatized after his father is extremely harsh on him for not following the family's strict rules. His akuma form, 'Rebel Son', might have a look that combines his Chat Noir features with a more rebellious and angry look. Marinette could be akumatized when she's framed for something she didn't do at school. Her akuma form, 'Innocent Victim', would have a look of someone who has been wronged and is out for revenge.
One possible story could be that Adrien gets akumatized because he feels constantly overshadowed by Ladybug. In this fanfiction, Hawkmoth might prey on his insecurities. As an akumatized form, Adrien could have enhanced physical abilities like super strength and speed. His akumatized look might be a dark version of his Chat Noir costume, with a more menacing appearance.
Well, obviously Marinette herself is a central character. Chat Noir is also very common as he usually has to deal with akumatized Marinette. Alya is often there too, as Marinette's friend. She might try to help Chat Noir understand what's going on with Marinette or try to find a way to reach out to her.
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.
One interesting plot could be Marinette being akumatized into a form that has the power to reverse the roles of heroes and villains in Paris. So, the akuma victims become the ones trying to save Ladybug and Chat Noir. Another plot might involve her akumatized form having the ability to create illusions of everyone's worst fears, which leads to chaos in the city as people are forced to face their phobias.
She might have a dark, twisted version of her usual pink and black color scheme. Maybe her pigtails would be wild and spiky, like they're full of dark energy. Her dress could be torn and tattered, with a more menacing style, and her eyes might glow a deep red color.
Nathalie might be a character in these fanfictions too. Given her role in Adrien's life in the original series, she could be involved in the events leading up to his akumatization. Maybe she accidentally triggers his negative emotions. And then there's Gabriel, Adrien's father. His actions or lack of actions towards Adrien could be a major factor in Adrien getting akumatized. So, he could be a key character in the fanfiction, either as someone who realizes his mistake and tries to help or as someone who makes the situation worse.
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.
Well, a plot could revolve around Adrien being akumatized when he discovers a dark secret about his mother. This could lead to his akumatized form having powers related to uncovering hidden truths and punishing those who have been lying. Or perhaps he's akumatized because he's tired of being in the shadow of Chat Noir. His akumatized self then tries to prove that Adrien is just as important as his superhero alter - ego. Also, he could be akumatized due to a betrayal from a friend, and his akumatized form focuses on isolating that friend from everyone else.