It could also be that Snape's claim is part of a larger scheme. Maybe he is trying to use Hermione's intelligence and magical abilities to defeat a new dark force. Hermione, in the process, might learn a lot from Snape about the darker aspects of magic and how to use them for good, which would be a major character development for her.
One possible plot development could be that Snape's claim over Hermione creates a great deal of conflict within the wizarding world. The Order of the Phoenix might see it as a threat or a strange alliance. Hermione herself could initially resist but then start to understand Snape's motives, which could be related to protecting her from a greater danger.
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.
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.
The plot could involve the mistletoe having some sort of magical effect. Maybe it makes Snape and Hermione relive certain memories. As they do, they start to understand each other better. For instance, they might see how they both overcame difficulties in their pasts, and this new understanding could bring them closer together.
For Hermione, she might develop a more empathetic side towards Snape. She could start to understand his past traumas and the reasons behind his cold exterior. This could make her less judgmental and more open - minded. As for Snape, he could soften up. With Hermione's influence, he might show his more vulnerable side and let go of some of his grudges, especially towards the students he disliked at Hogwarts.
Well, one plot development could be that Hermione is initially shocked but then realizes she has feelings for Fred too. They then have to deal with the reactions of their friends, like Ron being a bit miffed at first. Another could be that Hermione rejects Fred's proposal at first, but as the story progresses, she sees how much he truly loves her and changes her mind.
Well, perhaps his death in the Shrieking Shack could be part of a complex revenge plot. There could be an enemy who lures him there and manages to overpower him. This could be related to his past actions or allegiances. For example, if some Death Eaters still hold a grudge against him for his double - agent role, they might target him there.
In this fanfiction, it could also be that Harry's Gryffindor nature starts to rub off on Snape in some ways. They might have arguments about how things should be done, but through these interactions, Snape might start to see the value in Harry's way of thinking. And Harry, on the other hand, might learn some of Snape's more cautious and calculating ways, which could help him in his future battles.
Perhaps Snape discovers Hermione's extraordinary intelligence in a new way, which makes him excited. Maybe she solves a really difficult magical problem that he thought was unsolvable, and this newfound respect and admiration leads to his excitement.
Some fanfiction writers might do it for shock value. They want to create a very different and extreme situation compared to the original story to attract readers' attention.
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.