One great fanfiction is 'The Butterfly's Reign'. In this alternate universe, Hawkmoth has successfully won, and Ladybug and Chat Noir have to find a new way to fight back from the shadows. It's full of mystery and intrigue.
The main reason is the chance to explore different scenarios. Fans can see what would happen if the characters had different personalities or if the power dynamics changed. For example, in some alternate universes, Marinette might be more assertive as Ladybug from the start.
In an alternate universe where the Miraculous holders are more like superheroes in a traditional sense, Felix and Marinette could start as rivals. They both want to be the top hero in the city. But as they face bigger threats together, they realize they are stronger as a team. This mutual respect could turn into love over time as they see each other's true selves.
Perhaps he meets a version of his parents who are still alive. This would be an incredibly emotional encounter for Harry. He would get to know them, and they could teach him things about magic and life that he never had the chance to learn before.
One idea could be a castle that floats in the sky in the alternate universe. It could be made of some kind of magical crystal that gives it the ability to levitate. The inhabitants of this castle might be a race of winged beings who use the castle as a base to protect their realm from dark forces.
He might meet different versions of his friends. For example, a Hermione who is a powerful sorceress ruling a magical kingdom. It would be exciting as he could learn new magic from her and be part of her adventures in this alternate reality.
One possible alternate ending could be that the heroes discover a way to reverse the damage caused by Timetagger without having to fight him directly. Maybe they find an ancient artifact that has the power to reset the time changes he made. This would lead to a peaceful resolution and the restoration of the original timeline.
One idea could be a crossover where Ladybug and Cat Noir end up in the world of Steven Universe. They could team up with Steven and the Crystal Gems to fight a common enemy that has the power to disrupt both their universes.
This event in the fanfic could also lead to new characters emerging. Maybe someone would try to take on Ladybug's mantle in a different way. Or new villains could rise, seeing the opportunity in the chaos. The magic of the Miraculous might be affected too. Since Ladybug was an important part of the magical balance, her death could have unforeseen consequences on the powers and the Miraculous themselves.
The adult confidant in alternate universe fanfiction can act as a stabilizer. They can help the main character, who might be disoriented in this new world, to ground themselves. They are like a lighthouse in a stormy sea.