Marinette (as Ladybug) would be devastated. She might isolate herself at first, blaming herself for not being able to prevent it. Alya would be very concerned for Marinette and try to comfort her, but also be in shock at the loss of Chat Noir. Nino would probably be heartbroken as well, since he was friends with Adrian. The other classmates might be sad and confused, and there could be a general sense of gloom over the school.
One popular theme is alternate universes. Writers often explore what would happen if the characters had different backstories or powers. Another theme is relationship development, like how Marinette and Adrien's relationship could progress in different ways. And of course, there are action - packed themes where the heroes face new and more powerful villains.
Perhaps in the fanfiction, Adrian's death could be a result of a self - sacrifice. He might give up his life to save Ladybug or Paris from a great danger. This would lead to a whole new set of challenges for Ladybug as she has to deal with the loss of her partner and also continue to protect the city without him. His death could also be a catalyst for a new hero to emerge, or for Ladybug to gain new powers out of her extreme emotions.
For me, Alya is really interesting to write about in Miraculous Lady Bug fanfiction. She's Marinette's best friend and has her own ambitions and personality. She could be involved in the superhero world in different ways. Chloe can also be a great character to write about. You can explore her redemption arc or her continued antagonism in various storylines. And of course, Plagg, the Kwami of destruction, with his mischievous nature and unique relationship with Adrien.
I really like 'Felix's Redemption'. In this fanfic, Felix starts off as a rather antagonistic character, but as the story progresses, his relationship with Adrian evolves. It's interesting to see how they come to understand each other better, especially in the context of the Miraculous Ladybug universe with all its magic and mystery.
One popular theme is family secrets. Given their family background in the Miraculous Ladybug world, fanfictions often explore the secrets that Felix and Adrian might be hiding from each other or from the rest of the world. For example, there could be secrets related to their parents' past or the true nature of their family business.
Obviously, the Bad Bug would be one of the main characters. And probably the regular Ladybug and Chat Noir too, as they would be the ones to oppose the Bad Bug.
Ladybug and Chat Noir are extremely popular. They are the main characters in the original series, so in fanfiction, people love to write about their relationship, whether it's more romantic or just a strong friendship. Marinette is also popular as she has a complex personality and her own set of challenges.
Well, in Miraculous Lady Fanfiction, you can find stories that focus on the backstories of the villains. Maybe the reason Hawkmoth became so obsessed with the Miraculous has a more complex origin story in a fanfic. Also, there are fanfictions that give different endings to the main story, like Ladybug and Chat Noir finally revealing their identities in a more dramatic and unexpected way.
One idea could be exploring how the 'Bad Bug' version of the character interacts with the normal Miraculous Ladybug world. Maybe they cause chaos in different ways than the regular villains.