In this fanfiction, it could be that the Puppeteer has allied herself with other villains. Together, they form a powerful group that is more than a match for the Miraculous holders. The heroes would then have to figure out not only how to defeat the Puppeteer again but also how to deal with her new allies. Maybe Ladybug and Chat Noir would need to recruit more help or find new Miraculous powers to combat this threat.
A really cool plot could involve the Puppeteer teaming up with other villains in the Miraculous Ladybug universe. They could form a sort of 'villain alliance' and plan a major attack on Paris. The heroes would then have to figure out how to break their alliance and defeat them. Also, perhaps the Puppeteer discovers a way to use the Miraculous powers for herself without having a Miraculous, which would be a huge threat to Ladybug and Chat Noir.
One idea could be exploring the emotional aftermath for the characters after the events of Puppeteer 2. For example, how Ladybug and Chat Noir's relationship might have changed. Another could be introducing a new villain who takes advantage of the chaos left behind. Maybe a new Miraculous holder could also be introduced in the fanfiction, adding a new dynamic to the story.
One possible plot could be that Harry stumbles upon an old puppeteering book in the Hogwarts library and becomes fascinated. He then starts to create his own magical puppets which help him in his adventures.
Most likely, we'll see the Puppeteer as a central figure. Alongside her, Ladybug and Chat Noir are bound to be there as they always are to save Paris. Also, Alya might be involved as she often helps Ladybug. Since the Puppeteer has a history of using dolls, perhaps there could be a new character who is an expert in doll - making or some sort of antique doll collector who has some connection to the Puppeteer's powers or origin.
Another good pairing could be with Lila. Lila is a master manipulator herself, and when paired with the Puppeteer, they could create all sorts of trouble for Ladybug and Chat Noir. They could play off each other's devious natures and come up with some really elaborate plans to deceive the heroes.
Ladybug and Chat Noir are obvious choices. Their dynamic is always interesting to explore further. You can show how they grow and develop as heroes. Also, the Puppeteer herself is a great character to focus on. Her motives and powers make her a complex character that can be delved into deeper in fanfiction.
Well, it could be about Ladybug and Chat Noir finally revealing their identities to each other and having to deal with the consequences. Maybe they face a new, more powerful villain that is a threat to all of Paris and they have to use their combined powers in new ways.
Perhaps in the fanfiction, Nathalie gets a Miraculous that has a special connection to her past. As she uses it, she starts to remember things from her childhood that are related to the Miraculous. This could lead her to question her loyalty to Hawk Moth and start to work towards a more positive side.
In the fanfiction, Nathanael might get a miraculous related to his artistic talent. It could be a miraculous that enhances his creativity to superhuman levels. He could then use this to create amazing works of art that have some sort of magical or practical effect on the world around him, like inspiring people to be more kind or even physically changing the environment in a positive way.
One possible plot for a Miraculous Ladybug beach fanfiction is that Hawkmoth sends an akuma to the beach during a big surfing competition. Chat Noir is there to compete, and Ladybug has to show up and they end up having to fight the akuma while also trying to protect the civilians at the beach. They could also have some cute beach - themed banter in between the fight scenes.