A plot twist might be that the Peacock Miraculous starts to have an unexpected effect on Adrien's personality. Maybe it makes him more assertive in a way that his friends find a bit strange at first. He could start to make decisions that are out of character for him, like taking more risks in battles, which could lead to both positive and negative consequences. He might also develop a connection with the Miraculous that makes it difficult for others to use it when he's around.
One possible plot could be that Adrien stumbles upon the Peacock Miraculous by accident while exploring an old attic in his mansion. He then has to figure out what it is and how to keep it safe from Hawk Moth.
A plot could involve Adrien being given the Peacock Miraculous by a mysterious figure. With this new power, he tries to use it to protect a loved one from a new, powerful enemy. However, the power of the Peacock Miraculous starts to affect his emotions and he has to learn to balance his feelings while using the miraculous effectively.
In the fanfiction, it could be that Adrien's mom had a special bond with the Peacock Miraculous. She was able to use it in a unique way that no one else could. But using it also put her in danger. When she had Adrien, she knew she had to protect him from the threats associated with the Miraculous. So she disappeared, leaving behind clues for Adrien to find when he was old enough. As Adrien uncovers these clues, he gets closer to understanding his mom's connection to the Peacock Miraculous and the secrets she held.
With his friends like Nino, Adrien's use of the Peacock Miraculous could make them feel a bit left out at first. They are used to the 'normal' Adrien, and this new side of him with different powers could seem a bit alien. However, as they see him using the powers for good, they might become more interested and start to support him, but there could still be moments of misunderstanding. For example, if Adrien has to disappear suddenly to deal with a sentimonster, his friends might think he's avoiding them.
One possible plot could be that the peacock miraculous has a new holder. This new holder might be a bit of an anti - hero at first. They could use the peacock's powers to create sentimonsters for their own gain, like stealing valuable items. But as the story progresses, they start to see the harm they're causing and slowly turn towards the side of good, joining Ladybug and Chat Noir in their fight against Hawk Moth.
In a fanfiction, Adrien's mom might have used the Peacock Miraculous to create something special. For example, she could have created a magical object that was meant to protect Adrien when he was born. But then, due to some unforeseen circumstances, she had to disappear. This would tie in the Peacock Miraculous with Adrien's backstory in an interesting way. The story could then explore how Adrien discovers this connection as he grows older and becomes more involved in the Miraculous world.
One possible plot could be that Marinette, in an overzealous attempt to get closer to Adrien, creates a 'trap' of sorts. Maybe she arranges for a photoshoot at a location where only she and Adrien are present, under the guise of it being a school project related to fashion. She could have set up the set in a way that Adrien would be impressed by her creativity and they would have an extended period of time to talk and bond.
Well, the plot could involve Adrien initially being confused by the power of the Butterfly Miraculous. Maybe he accidentally activates it and starts sending out akumas without really understanding what he's doing. Then, he has to figure out how to reverse the chaos he's caused while also hiding his new - found power from his friends and family.
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.
You can try searching on fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. Just type in 'Adrien finds the Peacock Miraculous' in the search bar and browse through the results.