One possible plotline could be that Harry uses a powerful spell he's been secretly practicing to stun Bellatrix just in time as she's about to harm Hermione. For example, he might use a modified version of the Stupefy spell that's more powerful. Another could be that Harry manages to create a diversion, like summoning a horde of magical creatures, which distracts Bellatrix long enough for him to rescue Hermione.
Ron could be involved. He might be nearby when the situation occurs and try to help Harry in some way, like providing a distraction for Bellatrix. Neville could also be there. He's grown quite brave and might step in to assist Harry, perhaps by using his skills with Herbology to create some sort of plant - based defense against Bellatrix.
In some fanfictions, Bellatrix might save Harry because she is magically bound to him in some way. Maybe a spell backfired and now she has to protect him. Or perhaps she has a secret love for Sirius Black (Harry's godfather) and in order to honor his memory, she saves Harry. There could also be a plotline where Bellatrix is tired of Voldemort's tyranny and sees Harry as the one who can overthrow him, so she saves him for the greater good of the wizarding community.
One interesting plotline could be Hermione using her vast knowledge of magic to outwit a powerful dark wizard who has everyone else under a spell. She might find a long - forgotten spell in the library that can reverse the dark magic.
One possible plotline could be that Bellatrix, in a moment of unexpected self - reflection, decides to save Hermione from a dangerous dark magic experiment that the other Death Eaters are planning. Maybe she has a change of heart because she sees a bit of herself in Hermione's intelligence and determination.
There are fanfictions where Ron teams up with other characters to save Hermione from Bellatrix. Maybe he gets help from Neville, who has also grown to be quite powerful. Together, they manage to outwit Bellatrix. Ron could use his strategic mind to create a diversion while Neville sneaks up on Bellatrix and disarms her, allowing Hermione to be freed from whatever spell or trap Bellatrix had her in.
Another plotline might involve Harry adapting to Bellatrix's rather extreme and unorthodox parenting style. He could be learning dark magic in a way that still aligns with his own sense of good, which would create a lot of internal conflict for him. Bellatrix, on the other hand, might start to have a change of heart as she sees Harry's positive influence on her in some aspects.
In this fanfiction scenario, Bellatrix saving Harry from the Dursleys is quite an unexpected twist. It could be that Bellatrix has some ulterior motive. Maybe she wants to use Harry for her own dark purposes against Voldemort. She might see Harry as a valuable pawn in the power struggle within the Death Eater ranks.
Some fanfictions explore a different relationship dynamic between them. Maybe Harry has a problem that only Hermione can understand and help him with, and this brings them closer in a way that's different from their normal friendship. It could be something related to his connection with Voldemort or his family history.
In some fanfictions, Ron saves Hermione from Bellatrix in a more psychological way. He manages to get into Bellatrix's head, using a rare mind - influencing spell. He makes Bellatrix see her worst fears, which causes her to drop her wand and flee, leaving Hermione unharmed. This shows that Ron doesn't always have to rely on brute force to save Hermione. It also adds an interesting layer to his character as someone who can be clever and resourceful in different ways.
One possible scenario is that Snape steps in during a confrontation. Bellatrix is about to harm Hermione, and Snape, using his quick reflexes and advanced magic, stops her. He could be acting out of his own sense of right and wrong, or perhaps because he has a hidden agenda that involves keeping Hermione safe.