In some fanfictions, Ladybug might sacrifice herself to save Paris. For instance, if there is an unstoppable akuma attack that is about to destroy the city, she could use her powers in a way that she knows will kill her but will stop the threat. This would show her selfless nature as a hero.
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.
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.
I can't really recommend or go into detail about spanking - related plotlines as they may not be suitable or appropriate for all audiences. However, in general Miraculous Ladybug fanfiction might explore different relationships between the characters, like Ladybug and Cat Noir's partnership, or their individual struggles with their secret identities.
In some fanfictions, Marinette might fight against an alternate version of herself. This could be a version that has been corrupted or turned evil. It would be a really intense battle as she has to face someone with the same skills and knowledge as her. She'd have to dig deep and find the strength within herself to overcome this internal conflict.
In this fanfiction, Ladybug could be ill and in bed. Chat Noir, being the caring partner he is, tries to take care of her. He brings her soup (even though he's not very good at cooking) and tries to cheer her up. Ladybug, on the other hand, is a bit frustrated at being sick but is also touched by Chat Noir's efforts. They end up having a deep conversation about their feelings for each other, but still keep it within the boundaries of their superhero partnership.
A truth scenario could be that Ladybug has to tell Chat Noir the one thing she dislikes most about being a superhero. This could lead to a deep conversation about the sacrifices they both make and how they deal with the stress of protecting Paris. It would also show a more vulnerable side of Ladybug, which is not often seen.
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.
One possible plotline could be that an evil force is threatening both the Miraculous Ladybug world and the Voltron universe. So, Ladybug and Chat Noir are called upon to help the Voltron paladins. They have to learn how to use their powers in this new universe and also figure out how to combine their powers with Voltron's. Another plotline could be that a character from one universe accidentally ends up in the other. For example, Pidge could end up in Paris and meet Ladybug and Chat Noir. She could then try to find a way back home while also helping them fight Hawk Moth.
Maybe there's a breakdance competition in Paris and both Ladybug and Chat Noir enter it secretly. Along the way, they face challenges not only from their opponents in the competition but also from a supervillain who wants to disrupt the event. Their breakdancing skills become crucial in saving the day and winning the competition as well as protecting Paris.
One possible theme could be self - discovery. As Ladybug and Cat Noir grow, they might discover new things about themselves. Another theme could be friendship. Their relationship as superhero partners could be explored more deeply in terms of how it helps them grow.
One possible storyline could be that Ladybug sacrifices herself to save Paris from an ultimate villain. Maybe the villain has created a weapon that can destroy the entire city, and Ladybug uses her powers to contain the blast at the cost of her own life. Her death would then lead to a city in mourning and Chat Noir having to step up in a whole new way, trying to keep Paris safe without his partner while also dealing with his grief.