It could be that Harry has heard of a secret meeting or a gathering of some sort in Hogsmeade that he thinks Hermione should be part of. Maybe it's related to the fight against the Dark Arts. He believes her intelligence and quick thinking will be valuable in this gathering, and they can also gather more information together to take back to Hogwarts and share with their friends like Ron.
Perhaps Harry has a plan to cheer Hermione up. He asks her to Hogsmeade so they can visit The Three Broomsticks, have some Butterbeer, and just relax. He might sense that she's been stressed with all the schoolwork and he wants to give her a break in their favorite wizarding village. They could also stop by the Shrieking Shack and Harry could tell her some made - up spooky stories just for fun.
One possible scenario could be that Harry needs Hermione's help with something specific in Hogsmeade. Maybe he wants to research a certain magical item in one of the shops there, like Zonko's Joke Shop for some prank - related research or Honeydukes for information on a particular sweet that has a magical property. He knows Hermione is the best person to go with as she's so knowledgeable about all things magical.
One possible scenario could be that Hermione finally realizes her deep - seated feelings for Harry. In the fanfic, perhaps after a particularly dangerous adventure where they almost lost each other. She might approach Harry in a quiet moment, like in the Gryffindor common room late at night when most students are asleep. She could be a bit nervous but determined as she asks him out for a walk by the lake or a trip to Hogsmeade. This would be a huge moment for their relationship as it takes a more romantic turn.
One possible scenario could be that Hermione is initially shocked. She might have been expecting someone else to ask her, like Viktor Krum. But then she realizes that Harry is being sincere. She could accept his invitation, and they start preparing for the ball together, sharing nervous laughs and excited chatter about what they'll wear.
Well, Hermione might immediately say yes. She loves going to Hogsmeade and exploring all the magical places there. Once they get there, they could start by going to the bookshop. Hermione would be in her element, looking for new and rare magical books. Harry would probably follow along, maybe a bit less enthusiastic but still interested in whatever Hermione might find interesting.
It could be the Death Eaters. They are always looking for ways to cause chaos and undermine the wizarding world. Hogsmeade, being a place full of both students and ordinary wizards, would be a prime target for them to strike fear into the hearts of everyone. Another possibility could be some rogue dark wizards who are not directly affiliated with Voldemort but have their own evil agendas.
In some fanfictions, it might occur during a trip on the Hogwarts Express. They are sitting in a compartment, just the two of them. The train is chugging along, and the scenery outside is passing by. Harry looks at Ginny and starts a conversation about their future. Then he casually asks if she would like to go on a date with him when they get back to Hogwarts. It could also be after Ginny has been in some sort of danger. Harry is so relieved when she's okay that he blurts out his feelings and asks her out.
Perhaps Harry asks Padma for help regarding his date with Cho. He's all confused about how to make Cho happy at the ball. Padma, being a girl, might give him some useful tips. Like she could tell him about the kind of flowers Cho might like or the type of music she enjoys. This would show Harry's vulnerability and his attempt to make his date a success, while also creating an unexpected bond between him and Padma.
One possible consequence could be a lot of chaos and panic among the students and residents. With Hogsmeade being a popular place for Hogwarts students to visit, an attack would surely terrify them. Some students might get separated from their friends or get injured. The safety of Hogwarts itself could also be in jeopardy as Hogsmeade is so close. The local businesses in Hogsmeade would likely suffer great losses, and it might take a long time for the place to recover its former charm and prosperity.
Well, one possibility could be Itachi as a wizard in the Harry Potter world. Maybe he'd be in Slytherin due to his cunning nature. He could use his Sharingan abilities in a magical way, like seeing through invisibility cloaks or detecting hidden spells.
One possibility could be that they meet in a muggle setting, like a local park. Maybe Hermione is reading a book on a bench and Harry, who is always curious, approaches her to ask what she's reading. Another could be that they both end up at the same orphanage for a short time and form a bond over their shared sense of being different from the other kids.
It could be a new dark wizard who wants to make a name for himself. He sees Hogsmeade as an easy target to show his power. He starts by causing small disruptions like making objects in the shops move on their own and then escalates to more dangerous attacks that Harry has to stop.