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.
In some fanfictions, Adrian's death could be part of a complex time - travel or alternate reality plot. Maybe an enemy from the future comes back and changes the past in such a way that Adrian is killed. This would then set off a chain of events where Ladybug has to find a way to reverse time or enter the alternate reality to save Adrian. It could also involve her having to team up with different versions of characters she knows to achieve this goal.
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 possible plot could be that Ladybug and Chat Noir form a special 'Love Patrol' to protect the city during a Valentine's Day event. They notice that an akumatized villain is targeting couples and spreading hate. As they fight, they also start to realize their own feelings for each other more deeply. They use their powers in creative ways, like Ladybug's Lucky Charm creating heart - shaped objects that help them in the battle.
One possible scenario could be that Hawkmoth finally achieves his ultimate goal which leads to the death of Ladybug. Maybe he gets hold of both the Ladybug and Cat Miraculouses and uses their combined power in a way that causes a catastrophic event resulting in her death.
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.
Well, it could be about Ladybug and Chat Noir finally revealing their identities to each other and having to deal with the consequences. Maybe they face a new, more powerful villain that is a threat to all of Paris and they have to use their combined powers in new ways.
One possible plot for a Miraculous Ladybug beach fanfiction is that Hawkmoth sends an akuma to the beach during a big surfing competition. Chat Noir is there to compete, and Ladybug has to show up and they end up having to fight the akuma while also trying to protect the civilians at the beach. They could also have some cute beach - themed banter in between the fight scenes.
One possible plot could be that the peacock miraculous has a new holder. This new holder might be a bit of an anti - hero at first. They could use the peacock's powers to create sentimonsters for their own gain, like stealing valuable items. But as the story progresses, they start to see the harm they're causing and slowly turn towards the side of good, joining Ladybug and Chat Noir in their fight against Hawk Moth.
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.
Another aspect could be the discovery of new secrets. Maybe Master Fu left behind some hidden messages or items that the heroes need to find. This could lead them on a quest that uncovers more about the Miraculous and their history. It might also introduce new characters who are somehow connected to Master Fu's past.
Hawkmoth is an obvious choice. His whole mission is to obtain the Miraculouses, and in a death fanfiction, he could succeed in a more devious way than in the original series, directly or indirectly causing Ladybug's death.