Well, you could write a fanfiction where Dark Cupid accidentally infects one of the heroes, like Chat Noir, with her dark arrows. This would lead to some interesting internal struggles for the character as he battles his own warped sense of love. Or perhaps have Dark Cupid team up with another villain to create an even greater threat to Paris.
Use vivid descriptions. Instead of just saying Dark Cupid shoots an arrow, describe how the arrow glows menacingly, the sound it makes as it zooms through the air, and the look of determination on her face when she fires it. Also, add some twists to the plot. Don't just have the usual fight and victory. Maybe Dark Cupid has a hidden agenda that no one suspects, and it gradually unfolds throughout the story, keeping the readers on their toes.
Hawk Moth is often a very popular character in dark fanfiction. His villainous nature gives a lot of room for exploration in a darker context. Writers can delve into his motives more deeply and make him even more menacing.
One interesting story could be about a darker origin for the Miraculous. For example, the Miraculous were originally created for a more sinister purpose but over time were repurposed for good. In this fanfiction, Ladybug and Chat Noir might discover this dark past and have to deal with the moral implications.
In an akumatized Ladybug fanfiction, the akuma might enhance her powers in unexpected ways. For example, her Lucky Charm could become a more powerful and unpredictable object. Her akumatized form might also have a darker and more menacing look, perhaps with a more aggressive fighting style. Writers could explore how the heroes and villains interact with this changed Ladybug.
It could be a new character, like a dark sorcerer who wants to steal all the Miraculouses for his own dark magic. He could be specifically targeting the Dark Owl because he sees the Dark Owl as a threat to his plans.
In this 'Miraculous Ladybug Criminal Ladybug Fanfiction', there could be a lot of potential storylines. One possibility is that an external force is influencing Ladybug to commit crimes. It could be a new villain who has found a way to manipulate the Miraculous. This would create a great conflict as Ladybug has always been the symbol of good. Her friends and allies would be in shock and would have to find a way to stop her without hurting her. The story could also explore her internal struggle, as she might be aware of what she's doing but unable to stop herself.
A new villain could have a weapon that specifically targets Ladybug's powers. When she tries to use her yo - yo or her powers of creation, it backfires and causes her harm. This would force her to rely on her wits and Chat Noir more than ever. They would need to find a way to neutralize the weapon and heal Ladybug.