Maybe the troll is an illusion created by a powerful magical artifact. So when Harry tries to save Hermione, he discovers that the real danger is the artifact itself, which is draining the magic from the school. Or perhaps, the troll is a time - traveling version from the future, sent back to change the course of events, and Harry and Hermione have to figure out how to send it back without disrupting the timeline while still saving themselves.
One possible plot twist could be that the troll is actually under the control of a dark wizard. So when Harry saves Hermione, they realize that there is a much bigger threat at hand. Another twist could be that the troll turns out to be a transformed Hogwarts student, and they have to find a way to reverse the spell instead of just fighting it.
In this kind of fanfic, one possible scenario could be that Harry uses his quick thinking and magical skills. For example, he might use a stunning spell on the troll just in time to stop it from harming Hermione. Another scenario could be that Harry lures the troll away from Hermione, using his knowledge of the troll's behavior. Maybe he throws something noisy in the opposite direction to draw the troll's attention.
One reason could be that the troll is simply on a rampage and Hermione was in the wrong place at the wrong time. Trolls are not known for their intelligence, and it might have just stumbled upon Hermione while wandering the Hogwarts halls.
Well, it could start with Hermione being in a vulnerable situation, like being cornered by the troll in a dark, damp dungeon. Harry, being the brave hero, arrives just in time. He might first distract the troll by conjuring up some annoying little creatures that the troll would start chasing. Then, when the troll's back is turned, Harry would grab Hermione and apparate them both to a safer place, like the Gryffindor common room.
His courage becomes more evident. By saving Hermione, he shows that he is willing to face great danger for his friend. This act of heroism can make him more confident in his abilities as a wizard.
Well, in many such fanfictions, Harry might use his magic skills. He could cast a powerful stupefy spell on the troll to stop it in its tracks and then quickly lead Hermione to safety. Maybe he also uses his quick thinking and knowledge of the Hogwarts corridors to find a hiding place where the troll can't reach them.
Well, a plot twist might be that when Naruto arrives to save Hinata, he finds out that Hinata has already made a deal with the Kumo ninjas for the sake of Konoha. So Naruto has to figure out how to honor Hinata's decision while still protecting her. Also, it could be that the Kumo situation is just a cover - up for a much larger conspiracy involving other villages. And there's a chance that the person Naruto thought was Hinata in danger was actually a clone, and the real Hinata is in an even more dangerous situation elsewhere.
Another plot twist could be that Lucius's act of saving Hermione causes a rift between Hermione and Ron that is hard to repair. Ron might misunderstand Hermione's connection with Lucius and think she is now on the 'dark side' so to speak. This could lead to a complex love triangle situation where Hermione has to prove her loyalty to her friends while also dealing with her gratitude towards Lucius.
One possible plot twist could be that Naruto's closest friend turns out to be secretly working for the enemy. Just when Naruto is about to make a final stand against the invaders, his friend betrays him, leaving him in a difficult situation. But Naruto, being Naruto, manages to find a way to turn the situation around and still save the village.
A possible plot twist could be that Bardock actually saves Broly, but in doing so, he unknowingly sets in motion a chain of events that leads to an even greater threat. For example, by saving Broly, he might disrupt the balance of power among the Saiyans in a way that angers some powerful factions, who then form an alliance with an external enemy.
Perhaps in this fanfic, Severus doesn't just save Harry physically from the Dursleys. He also saves Harry from the emotional and psychological damage that the Dursleys have been inflicting on him. Severus, having his own traumatic past, might be able to relate to Harry on a deeper level and starts to mentor him in ways that Dumbledore never could. This could lead to a completely different Harry, more confident and less impulsive.