Another possibility could be a new character introduced specifically for the fanfiction. Maybe a powerful and charming hero from another city comes to Paris to help Ladybug and Chat Noir. He and Ladybug develop a connection, and he could end up being the father. This would create a lot of drama and new storylines, especially when it comes to how Chat Noir reacts to the situation.
If the fanfiction takes a really wild turn, it could be that the father is a sentient Miraculous. I know it sounds strange, but in the world of fanfiction, anything is possible. Maybe the Miraculous has some sort of power that allowed it to impregnate Alya in a very unique and magical way, and the story then delves into how Alya and her friends deal with this extremely unusual situation.
It could be the Mad Father himself. If he were to find out about the Miraculous and try to use them for his own twisted experiments, he would make a great antagonist. He could try to capture Ladybug and Chat Noir to study their powers.
Well, it could be about Alya dealing with the challenges of being pregnant while still being involved in the superhero chaos. Maybe she has to keep it a secret from the villains who might use it against her. Or perhaps she discovers some new powers related to her pregnancy that help Ladybug and Chat Noir in their battles.
It could be Alya. She's very curious and adventurous. Maybe she gets too close to some sort of werewolf lore or a werewolf in disguise while investigating for the Ladyblog. And then she gets bitten and turns into a werewolf. This would add a whole new layer of drama to her relationship with Marinette and the other characters.
In this fanfiction wedding, not only their friends but also some of the heroes they've worked with in the past could be guests. Rena Rouge and Carapace would be there, having played important roles in many battles. Maybe even some of the one - time heroes like Pegasus or Viperion could show up. Ladybug's parents would be there with open arms, and Adrien's bodyguard could also be present, having seen Adrien grow up. And Paris itself could be considered a 'guest' in a way, as the city has been a big part of their lives and the reason for their superhero existence.
In a Miraculous Ladybug sleepover fanfiction, the characters might have a lot of fun. They could play games like truth or dare, which could lead to some interesting revelations. For example, Marinette might accidentally let slip some of her feelings for Adrien.
The girl might be a new superhero in training. She comes to Paris to learn from Ladybug and Chat Noir. During her time there, she catches the eye of one of them, and the story of'marry that girl' begins. She could bring a fresh perspective to the Miraculous team and also add a new dynamic to the relationship in the fanfic.
A villain that has the power to steal emotions could be a great choice. Imagine this villain targeting Edendaphne's friends and allies, draining their positive emotions. Edendaphne would have to find a way to not only stop the villain but also restore the emotions of those affected. It would be a very emotional and intense story.
A new character could be introduced as the main villain. Maybe a corrupt police officer who turns to the dark side. He could use his knowledge of the law and the city's security systems to commit crimes and frame the Miraculous heroes. He would be a formidable opponent as he has insider knowledge.
An evil alien warlord could be the main villain. This warlord could be bent on conquering Earth and other planets in the universe. He might see the Miraculouses as a threat and try to destroy Ladybug and Chat Noir first.