In a fanfiction, the plot could revolve around Bellatrix misusing the Cruciatus Curse and getting in trouble with Voldemort. She uses it on one of Voldemort's favored servants by mistake. Voldemort then has to decide whether to punish Bellatrix severely or let her off with a warning, while Bellatrix tries to plead her case and shift the blame onto others.
A fanfiction might explore the backstory of Bellatrix's use of the Cruciatus Curse. Perhaps she was first introduced to it as a young witch at Hogwarts by some dark wizarding group. She practiced it on small animals, which led to her getting in trouble. But this early experience with the curse made her more interested in the Dark Arts, and she continued to perfect her use of Crucio as she grew older and became a Death Eater.
One idea could be a story where a young Death Eater in training is forced to use the Cruciatus Curse on an innocent by his family, but he has a change of heart. He then has to deal with the consequences of defying his family's wishes and trying to protect the victim from further harm.
The plot could revolve around Bellatrix's memories in Azkaban. Since she was there for a long time, she might have had some extremely vivid and disturbing memories. A fanfiction could explore these memories and how they influenced her actions after she escaped. For example, she might have seen the faces of those she killed in her dreams while in Azkaban, and this could have made her more vengeful.
Show the physical and mental toll it takes on the victim. For example, describe how the victim's body contorts painfully and their mind is filled with unbearable agony. Maybe the victim has nightmares about the curse long after it has been used on them.
A plot might involve Harry being captured by Bellatrix. Instead of harming him immediately, she is intrigued by his power. She tries to break his spirit but Harry resists. Over time, through their power struggles and intense conversations, a strange connection forms between them that changes their relationship in unexpected ways.
In a 'Harry Potter Saves Bellatrix' fanfiction, perhaps Bellatrix is in a situation where she's about to be killed by other Death Eaters who think she's a liability. Harry, having a sense of justice and seeing the potential for redemption in her, steps in to save her. He could then try to show her the error of her ways, using his own experiences of losing loved ones to Voldemort's ideology. Bellatrix might initially resist, but gradually come to understand Harry's point of view.
One possibility is that in the fanfiction, the use of the Cruciatus Curse on Hermione could be a central plot point for her character development. Maybe she endures it and it makes her stronger, more determined to fight against dark forces.
Well, perhaps the Weasley family takes in a new house - elf that was previously under the Imperio curse by an unknown wizard. As the family tries to rehabilitate the house - elf, they discover that the curse has left some lingering effects. The Weasley children, with their knowledge of magic from Hogwarts, try different spells and potions to fully free the house - elf. Meanwhile, they have to be on guard as the wizard who cast the Imperio curse may come looking for the house - elf.
One idea could be a time - travel fanfiction where Hermione goes back in time and has to face Bellatrix in a magical duel. Harry could be caught in the middle, trying to protect Hermione. Maybe Bellatrix has a secret that could change the course of the wizarding war, and Hermione discovers it.
Well, perhaps Bellatrix was under some sort of spell or coercion when she had Harry. In the fanfiction, Harry might start having strange visions or powers related to Bellatrix's dark magic. As he enters Hogwarts, he is constantly suspected by Dumbledore and the Order of the Phoenix because of his connection to Bellatrix. But Harry, being Harry, still wants to fight against Voldemort, which creates a complex web of relationships and power struggles within the wizarding world. He may also try to find out more about his mother's past and what really happened when he was born.