One possible plot could be that Bellatrix is trying to intimidate Hermione. In many fanfictions, Bellatrix is a menacing character. She might straddle Hermione's waist to assert her power and dominance, perhaps during an interrogation where she wants to break Hermione's spirit and get information from her.
Maybe it's part of a magical duel or encounter gone wrong. Bellatrix could have overpowered Hermione during a fight and this is her way of showing she has the upper hand. It could also be a situation where Bellatrix is under the influence of a dark spell or potion that makes her act more aggressively towards Hermione.
In some fanfiction concepts, Bellatrix might be acting on behalf of Voldemort. She straddles Hermione's waist as a way to send a message to the Order of the Phoenix that they can get to their members easily. It could also be a setup for a later rescue or escape sequence where Hermione's friends come to her aid while in this vulnerable position.
Chapter 519, "I'm Here to Participate in the Program", is the clearest early answer because it turns Divine Son's situation into a concrete plot event. It opens with Seeing that everyone who went to attack Jiang Li had failed...
Chapter 658, "What About My Daughter-in-law?", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event. Early on, Jiang Li remained calm and composed, "You didn't ask me, did you?"....
Chapter 366, "The Safety Lock Got Flattened?!", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event. The chapter turns that into something concrete by moving from You're finally out!" Xiao Wu saw Jiang Li and the others from a...
Chapter 556, "Five-Year College Entrance Exam, Three-Year Simulation", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event. A useful checkpoint is the way the scene starts with After finally settling down, it wasn't long before Jiang Yan started...
Chapter 481, "She Actively Pursued the Boss", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with Bai Lu ultimately couldn't exchange for 500 points. and closes on Lili, what do you need these items for?" Jiang Li took out one of the....
Chapter 849, "Observing Fate Through Meridians", is the clearest early answer because it turns Miss Rong's situation into a concrete plot event. It opens with Gu Qiang slumped on the ground, somewhat fearfully avoiding Zhao... and closes on She smiled slightly, lifted her skirt, and walked over..
Chapter 652, "He's Gu Shengrong's Grandson?!", is the clearest early answer because it turns Gu Yu's situation into a concrete plot event.
Chapter 729, "Beauties Often Have Tragic Fates", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from In the deep of winter, the Capital experienced several heavy...
Chapter 658, "What About My Daughter-in-law?", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. It opens with Jiang Li remained calm and composed, "You didn't ask me, did you?"...
Chapter 479, "Unrivaled!", is the clearest early answer because it turns Deserted Island's situation into a concrete plot event. Early on, Brother Pig, who had been tied up for a day, was very angry, and the.... By the end, I was the one who tied up the wild boar in the first place,....