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.
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.
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.
Once upon a time, Marinette and Adrien's big day finally arrived. The city of Paris was adorned with flowers and ribbons. Marinette, in her beautiful white gown, walked down the aisle. Adrien, looking dashing in his suit, had tears of joy in his eyes. Their friends and family watched with love. As they said their vows, the whole place was filled with an air of magic. And then they were pronounced husband and wife, sharing a passionate kiss as confetti rained down on them.
A great element in such a fanfiction could be the family aspect. Maybe they start thinking about having children. How would they raise their kids? Would their children inherit their superhero powers? And how would they protect their family from the villains in their world? Their friends like Alya and Nino could also play important roles in their married life, perhaps as godparents or just as good friends who are always there to offer advice and support.
Yes, there are plenty. There are fanfictions that focus on their budding romance, like the ones where they go on a cute date or have a heartfelt conversation that deepens their relationship in a sweet and innocent way.
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.
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.