He might introduce more fair lending practices. In the wizarding world, some families or individuals might have been unfairly treated when it comes to borrowing money from the bank. Harry could change that.
One common theme could be the exploration of an unlikely relationship. Since trolls are not typical romantic partners in the Harry Potter universe, it's about how such a relationship might develop. Another theme could be the contrast between the two characters' natures, like the brutishness of a troll and the more complex personality of a Harry Potter character.
A powerful dark wizard who has infiltrated the Troll Market and is using it as a base to gather dark magic items. He could be after the Elder Wand or some other powerful artifact from the Harry Potter universe and is using the chaos and secrecy of the Troll Market to his advantage. He would see Harry Potter as a threat to his plans and try to eliminate him.
One possible plot could be that Harry Potter, while on a mission for the Order of the Phoenix, accidentally stumbles into the Troll Market. There, he discovers a magical artifact that Voldemort is also seeking. With the help of some of the unique creatures in the Troll Market, like the goblins or trolls who have their own brand of magic, Harry has to outwit Voldemort's followers who are also searching for the artifact.
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.
Since I don't have access to the 'harry bribes the troll fanfic', I can only make assumptions. It could be that the troll is blocking Harry's way to something important, like a secret passage or a magical object. Harry, being resourceful, decides to bribe the troll. He might offer it food that trolls like, such as a large amount of meat or some sweet magical treats. This bribe could then change the troll's behavior and set off a chain of events in the story. It could also explore themes like Harry's morality in using bribery to achieve his goals.
Another reason for its popularity is that it gives fanfic writers a chance to be really creative. They can create unique troll characters with their own personalities and senses of humor. And when these trolls interact with Harry, it can lead to all kinds of wacky storylines. Also, it's a break from the more serious and often dark themes in the original Harry Potter series, providing a fun escape for readers.
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.
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.