One possible plot development could be that Hermione manages to use her wits even in this dire situation. For example, she might use non - verbal magic to send a signal for help. Another could be that Bellatrix is using Hermione as bait to lure Harry or Ron into a trap. Maybe Bellatrix is trying to extract information from Hermione about the Order of the Phoenix.
In this kind of fanfiction, the plot could be that Bellatrix has Hermione bound and gagged as a form of revenge for Hermione's intelligence and her role in fighting against Voldemort. Hermione could try to free herself by using her magical skills in a very subtle way, like using her wandless magic that she has been secretly practicing. The story might also involve the other Hogwarts students or professors finding out about Hermione's situation and mounting a rescue mission. This could lead to a big battle where the good guys try to defeat Bellatrix and her Death Eater allies while also rescuing Hermione.
I'm not sure if there is such a specific fanfiction. Fanfiction covers a wide range of themes and scenarios, but this one seems rather dark and not in line with the general positive spirit of the original story. However, the fanfiction world is vast, and it's possible that something like this exists in some corner of the internet.
Hermione might also develop a different perspective on power. When bound, she is in a powerless state. This could make her re - evaluate how power is used in the wizarding world, and she could become more empathetic towards those who are oppressed. In addition, she may become more creative in her problem - solving skills, as the usual magical solutions might not be available to her while bound.
Another possibility is that her growth spurt could lead to some comical situations. Maybe she can't fit into her old robes anymore and has to constantly adjust them during classes, which becomes a running joke among her friends. Also, it could affect her Quidditch skills if she decides to play. A taller Hermione might have an advantage in reaching for the Snitch or blocking opponents.
One possible plot could be that the two lovers are in a dangerous situation where they are bound and gagged by some villains. But their love for each other gives them the strength to endure and eventually find a way to escape and be together.
Perhaps Bellatrix kidnapped Hermione to use her as bait to lure Harry. Bellatrix has always been extremely loyal to Voldemort and might think that by having Hermione, she can draw Harry out into the open. Hermione, being the intelligent witch she is, could try to outwit Bellatrix while in captivity, maybe by finding a way to send messages to her friends or even by trying to escape on her own.
One possible plot development could be that Sirius becomes a sort of protector for Hermione. He might use his knowledge of the wizarding world and his own skills to keep her safe from various threats. For example, if there are dark wizards after Hermione due to her intelligence or her connection to Harry, Sirius could step in and fend them off.
Another development might be the reaction of their friends. Harry and Ron might be initially skeptical or even opposed to this situation. There could be a lot of tension within the group as they try to come to terms with what it means for Hermione to be claimed by Remus. It could also lead to some rifts in their friendships at first.
There might be a fanfiction where Bellatrix gags Hermione during a battle at Hogwarts. Hermione, while gagged, uses her magic in a creative way to signal her friends for help. This shows her resourcefulness even in such a difficult situation.
One possible plot could be that Bellatrix captures Hermione and tries to torture her for information about Harry. But Hermione, using her wits and some new-found powerful magic she has been secretly studying, manages to turn the tables and kill Bellatrix in self - defense.
It could be a thriller where the woman in leather has some mysterious motives for binding and gagging someone. Maybe she's a villain in a heist story, trying to keep someone quiet who has discovered her plan.