One possible plot could be that Hermione is attacked by a dark wizard while on a solo mission to retrieve a powerful magical artifact. She gets seriously injured, and her friends have to race against time to find a cure. Another could be she's caught in the crossfire of a battle between rival magical factions at Hogwarts, leaving her badly hurt and the school in chaos as they try to protect her and find out who's responsible.
Ron would be devastated. He might initially be at a loss for what to do. But then his Gryffindor courage would kick in. He'd start researching every possible cure or treatment for her injuries. He'd be the one to comfort Harry and the others while also being the most vocal in demanding justice for Hermione. His relationship with Hermione, whether you see it as just friends or something more, would make him act in a very passionate and protective way.
One possible plot development could be that his friends rally around him. They might search for rare potions or spells to heal him. For example, Hermione could be scouring the library for ancient healing magic while Ron stays by Harry's side, offering moral support.
In some fanfictions, Hermione gets injured in the Forbidden Forest. Maybe she's exploring and stumbles upon a dangerous creature or a dark magic trap. This kind of plot allows for a lot of adventure and mystery. It also gives the author a chance to show how she copes with the injury in a dangerous and unfamiliar environment. And sometimes, she gets injured in a magical accident in the library while researching some dark magic, which can lead to all sorts of interesting consequences and character development.
One plot could be that Harry is seriously injured during a battle with a new dark wizard who has emerged. His friends then have to find a rare magical herb to heal him while fending off attacks from the wizard's followers. Another could be that he gets injured in a mysterious accident during a dangerous potion - making experiment at Hogwarts, and it's up to Hermione to figure out the antidote before it's too late.
Perhaps Ginny gets injured in a fight against some dark wizards. Her family and friends are distraught and they all try to contribute in her recovery. Some might use magic to heal her physical wounds while others try to keep her spirits up. Or, she could get injured during a magical experiment gone wrong in Hogwarts. Her classmates then come together to figure out how to reverse the effects of the accident that caused her injury.
Well, a plot might involve Paul being injured while trying to catch a very rare and powerful Pokémon. His injury makes it difficult for him to continue his journey, but he's determined not to give up. His friends or fellow trainers might come to his aid, and in the process, he learns about the importance of teamwork and friendship, which is a departure from his usually solitary and cut - throat style. Maybe he also has to train his Pokémon in new ways while he's injured to compensate for his own limited mobility.
One possible scenario could be during a battle at Hogwarts. Maybe the Death Eaters target her specifically, and she gets hit by a dark curse while trying to protect her friends. Another could be in an accident in the Forbidden Forest. She might be exploring and stumble upon some dangerous magical creatures, like a centaur gone wild, and get injured in the encounter.
One possible plot could be that the Howler is a warning about a dangerous magical experiment Hermione is about to undertake. It scolds her for not considering the potential consequences and warns her to stop. This would put Hermione in a difficult position as she has to decide whether to heed the warning or continue with her pursuit of knowledge.
Well, it's a rather dark concept. One possible plot could be that in a dark, alternate universe, Harry is under some sort of dark magic influence which makes him act against Hermione's will. But this is very much against the essence of their characters in the original series.
One possible plot could be that they are in arranged relationships with other people, but they start to have feelings for each other secretly. And then they have to hide their attraction while being unfaithful to their partners.