One possible plot could be that Adrien discovers some dark secrets about his family or the superhero world. Maybe he feels betrayed by Ladybug or the other heroes. This disillusionment could push him towards the dark side. For example, if he finds out that his father was a victim of the so - called 'good' superheroes' actions in the past, he might want revenge.
It could start with Adrien being framed for something he didn't do by a powerful villain. As a result, he is shunned by society and the superhero community. In his desperation to clear his name and get revenge on those who wronged him, he turns to dark magic or powers. Slowly, his once - noble intentions become twisted, and he becomes a villain, using his charm and intelligence in a malicious way.
It could be Ladybug. If Adrien turns villain, Ladybug would be the natural antagonist as she represents the good side. Their relationship would be completely flipped from what it was before. She would have to try to stop him from causing more harm, all while probably still having some feelings for the old Adrien.
Perhaps the cat tail is a curse at first. Adrien has to deal with all the awkwardness it brings, like not being able to hide it properly. But as time goes on, he realizes that the tail gives him an edge in certain sports or activities, like climbing. So it turns from a curse to a sort of advantage.
If Adrien becomes Hawkmoth in fanfiction, Paris would be in for a major shock. His friends, like Nino and Alya, would never suspect him at first. And Marinette, who is Ladybug, would be facing a completely different kind of enemy. She's used to fighting Hawkmoth as this unknown figure, but now it's someone she knows so well. It would change the dynamic of their battles entirely.
In a 'Danny Phantom becomes villain' fanfiction, it could be that Danny has a major falling out with his friends and family. He feels betrayed and abandoned. This emotional turmoil causes him to snap and turn to the dark side. He then uses his ghost powers to take revenge on those who he thinks have wronged him, becoming a feared villain in the process.
In a fanfiction where Naruto becomes a nukenin, it could be due to a prophecy that the village misinterprets. The village elders think Naruto will bring destruction, so they try to imprison or kill him. Naruto, not wanting to be a victim, escapes and becomes a nukenin. He then travels around, learning new jutsu from other lands and becoming stronger. Eventually, he might come back to the village to show them that their fears were unfounded, but by that time, he has become a very different ninja, with a new set of values and powers.
One possible plot could be that Adrien has been through some sort of difficult situation, like being trapped in a cold place for a long time, which has made him limp and still cold. His friends or a special someone then tries to nurse him back to health and warmth.
It could also be due to a spell or an akuma's special ability. In the Miraculous universe, there are many powers that can immobilize characters. For example, if an akuma had the power to paralyze people, Adrien might have been caught in it. And since he's often in the middle of the action, he'd be a prime target. He might be motionless as his body is fighting off the effects of this external force, trying to break free from it so he can transform into Chat Noir and save the day.
One possible reason could be that Marinette has been acting strangely in ways that seem to Adrien to be related to Hawkmoth. Maybe she has been seen in places or at times that are suspicious, like near akuma attacks or where Hawkmoth might be operating from. Another reason could be that someone has been spreading false rumors about Marinette, leading Adrien to believe she has some connection to Hawkmoth.
In a 'naruto becomes a missing nin fanfiction', perhaps Naruto discovers some dark secrets about the village leadership. These secrets are so dangerous that the village wants to silence him. He then decides to go rogue to expose these secrets to the world. As a missing nin, he might form alliances with other outcasts who also have a grudge against the current system.
Well, one possibility is that Naruto feels betrayed by the Hidden Leaf Village. Maybe they wrongly accused him of a serious crime. So, he decides to go rogue. He could then start traveling alone, using his skills to survive and avoid capture. He might also meet other outcasts who understand his situation.