Hawk Moth can also be a main character in such fanfictions. His reaction to the reveal of Ladybug and Chat Noir's identities is important. He might try to take advantage of the situation, or be forced to change his plans completely. And his presence adds an element of danger and conflict to the story.
The main characters are Ladybug and Chat Noir. But don't forget about Hawk Moth, the villain. In some fanfictions about the big reveal, Hawk Moth might find out their identities during or after the reveal, which changes the whole dynamic of the story. He could use this knowledge to try and defeat them in a more devious way, so his presence is still very much a part of the plot even though the focus is on the reveal between Ladybug and Chat Noir.
Most likely Ladybug and Chat Noir would be the main characters as they are central to the Miraculous Ladybug series. They would probably be the ones whose identities are at risk of being revealed during the dodgeball game.
One possible plotline could be that Ladybug and Chat Noir's identities are revealed during a major battle against a powerful akuma. This causes chaos at first as the citizens of Paris are shocked, but then they quickly come together to support the heroes in a new way.
Naruto is usually a main focus. Since the concept is related to perfection, his journey to become the best Hokage and the strongest ninja is often explored in greater detail. He might be shown overcoming his flaws more quickly or mastering new techniques faster. Sasuke is also likely to be a central character. His path of revenge and his own pursuit of power can be re - imagined with the idea of perfection, like achieving the ultimate Sharingan or new levels of his combat skills.
Often, Bella is still a central character. Even in fanfictions about moving on, her journey is important as she might be dealing with the aftermath of her relationship with Edward or Jacob. Her growth and how she adapts to new situations are key.
Sometimes, even the family pets or the neighbors can be focused on in fanfictions related to this episode. They could be the ones who help Lincoln figure out what's really going on with his bad luck or they could be affected by his luck in some comical ways.
Sonic, Sonia, and Manic are usually the main focus. They are the core of Sonic Underground and in the reunited fanfiction, their relationships and how they come back together are often explored.
Ruby is often a main focus. Her leadership and combat skills can be explored in new ways in the 'Warm Bite' setting. She might be the one leading the charge against new threats.
Jacob can also be a main character in this type of fanfiction. His unique position as a werewolf and his relationship with Bella and the vampires make him an interesting character to explore in the context of changing tides. He might be trying to protect his tribe in a new situation or deal with his feelings for Bella in a different way.
Ruby is often a main focus. Her bright and positive nature can be explored more deeply in the context of these coloured dreams. Her relationship with her scythe and her journey as a leader can take on new twists in the fanfiction.