Well, a possible plot could be that Marinette is a transfer student to Gotham. Her sunny personality stands out in the dark city. She could start a small business related to fashion and use it as a front to help the less fortunate. As she gets more involved in the city, she catches the attention of Batman and his allies. They initially suspect her but soon realize she has good intentions and she ends up working with them to take on a new supervillain who is targeting the city's art scene.
One idea could be Marinette moving to Gotham and using her unique skills to help the city in a different way than the local heroes. For example, she could use her fashion design skills to create protective gear for the people in dangerous areas. Another plot could involve her befriending some of the Gotham characters like Barbara Gordon and sharing her positive outlook, which gradually changes the gloomy atmosphere of Gotham. She might also get involved in some of the city's mysteries and use her cleverness to solve them.
One main antagonist could be Scarecrow. His fear - inducing tactics could be used against Marinette and the people she's trying to help. He might see her as a threat to his plans of spreading fear throughout Gotham. Also, Black Mask could be an antagonist. He would likely oppose Marinette if she tries to clean up the criminal underworld in Gotham, as it would cut into his business and power.
A great plot for Gotham Academy fanfiction could be a story centered around a school competition that turns out to be rigged. The main characters suspect foul play and start their own investigation. Or perhaps a story where a character from a villain's family enrolls at the academy and has to deal with the prejudice from other students while also trying to prove they're not like their family. Also, a plot about a haunted dormitory at Gotham Academy would be really cool. The students would have to figure out the mystery behind the hauntings.
A great plot could be that the male OC is a street magician in Gotham. One day, he discovers that his magic tricks are actually real magic due to an ancient artifact he found. He then uses this magic to fight crime, but he has to keep it a secret from the authorities and the criminal underworld. Or perhaps the male OC is a detective who has a unique ability to see the memories of the dead. This helps him solve crimes that no one else can, but it also takes a toll on his mental health as he is constantly exposed to the trauma of the victims. Another plot could be that he's a young heir to a wealthy family who decides to use his family's resources to fight against the poverty and crime in Gotham instead of living a life of luxury.
Perhaps there's a villain in Gotham who has discovered a way to siphon the Flash's speed force. The Flash has to go to Gotham to stop this villain, and in the process, he meets the various characters of Gotham like Gordon and Catwoman. They all have to work together to not only stop the villain from stealing the speed force but also from using it for some evil plan in Gotham.
Maybe there's a meta - human or a super - villain who is causing chaos in both cities. Batman is too occupied with Gotham's internal affairs, so Arrow comes to help. They discover that the villain has a base in an abandoned area between the two cities. They have to combine their resources, like Arrow's high - tech arrows and Batman's detective skills (even if Batman himself isn't directly involved but his influence on Gotham's crime - fighting is there), to take down the bad guy.
Well, the plot could start with Marinette being excited for the trip but also a bit worried due to Gotham's dangerous reputation. Once in Gotham, her class visits various places like Arkham Asylum for an educational tour. However, things go wrong when some of the inmates escape. Marinette uses her skills, both in combat (from her Ladybug training) and in persuasion, to keep her classmates safe and help the Gotham police recapture the inmates. Meanwhile, she might start a friendship with a Gothamite who shows her the real heart of the city beneath all the chaos.
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.
Here are some plot ideas. Felix and Marinette could be paired up for a dance competition at school. At first, they are both reluctant, but as they practice, they develop feelings for each other. Another idea is that there's a new villain in town who targets Marinette specifically because of her connection to the Miraculous. Felix, being the observant person he is, notices the strange things happening around Marinette and decides to help her. He might even pretend to be her bodyguard. And finally, a story where Felix and Marinette accidentally swap bodies due to a magical mishap. They have to figure out how to reverse it while also dealing with each other's lives and responsibilities.
Maybe their child gets sent back in time to when Adrien and Marinette were still just friends and classmates. The child then has to figure out a way to get back to their own time without changing the past too much. They might accidentally cause some comical misunderstandings between their parents - to - be in the process.
One idea could be exploring the backstories of the Gotham Knights more in - depth. For example, you could write about how they trained before becoming the Knights. Another idea is to create new villains for them to face. Maybe a villain with the power to manipulate technology in Gotham. Also, a fanfiction could focus on the relationships between the Knights, like how they handle disagreements within the team.