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.
The main characters would be the Puppeteer, Ladybug, and Chat Noir. But it's also possible that other Miraculous holders like Rena Rouge could make an appearance. Given that the Puppeteer is all about controlling things, there might be a character who has the power to resist her control, like a new Miraculous holder with a power related to mental strength or freedom. This character could play a crucial role in the story.
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.
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.
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.
The main characters would definitely be Marinette and Adrien. Marinette, who is Ladybug, would be a central figure as she tries to balance her normal school life during the trip with her superhero duties. And Adrien, as Chat Noir, would be there to support her and also have his own storylines, like dealing with his overbearing father's restrictions even during the trip.
Most likely, Ladybug and Chat Noir will be the main characters involved. Since they are the central figures in the 'Miraculous Ladybug' series, it makes sense that they would be the ones in this '7 Minutes in Heaven' scenario. They have a complex relationship filled with mystery and attraction, which would make for an interesting fanfiction.
Marinette and Adrien are also very likely to be in it. Since they are the civilian identities of Ladybug and Chat Noir respectively, they could be at the beach with their friends, like Alya and Nino. Maybe Alya could be trying to get some juicy photos for the Ladyblog while at the beach.
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.
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.
Obviously, Ladybug and Chat Noir are the most likely. They already have a complex relationship, and a love potion could really stir things up between them. Marinette, as Ladybug, could be the one accidentally affected by the potion and start acting strangely around Adrien, who is Chat Noir. Another character could be Alya. She's Marinette's best friend, and she could get caught in the middle of the love - potion - induced chaos. Maybe she tries to help Marinette figure out what's wrong or even gets affected herself and falls for Nino in a more extreme way. Hawk Moth is also a candidate. He could use a love potion as part of his evil plan to manipulate Ladybug or Chat Noir.
Obviously, Marinette and Adrien would be key characters. Marinette could be the host of the sleepover, and Adrien might show up unexpectedly, either as himself or as Chat Noir. Their interactions would be the focus, whether it's some cute and awkward moments between them.