One possibility could be a spell gone wrong. Maybe some dark magic or a misfired charm during a chaotic event in the wizarding world caused their minds to be altered and they unknowingly got married. Another idea is that it was part of a devious plot by someone who wanted to stir up trouble between Gryffindor and Slytherin. It could also be a result of a strange magical contract that they were unknowingly part of.
Harry would probably be shocked and a bit suspicious at first. He'd wonder if Draco had some underhanded plan. Ron, on the other hand, would likely be outraged. He's always had a bit of a rivalry with Draco and the thought of Hermione being married to him would be hard for him to accept. Ginny might be more open - minded and try to figure out what really happened.
He could be married to Luna Lovegood. Their relationship in the original series was more of a friendship, but in fanfiction, it could be that they got married. Maybe they were on an adventure together, something went wrong magically, and they woke up married. Luna's unique personality would bring a lot of interesting elements to the story as they figure out their new relationship.
In this kind of fanfiction, Ash might wake up with new Pokemon - like abilities. For example, he could communicate with Pokemon in a more direct and profound way, like understanding their every thought and emotion just by looking at them. He might also find himself physically changed, perhaps having some physical features of a Pokemon, which could lead to all sorts of interesting adventures and interactions with his friends and other Pokemon trainers.
It could start with an accidental encounter in a less - traveled part of Hogwarts. Maybe they both discover a hidden room or a secret passage. In that moment of shared discovery, they put aside their differences. As they start to explore the mystery together, they begin to understand and appreciate each other. Hermione might find Draco's sense of adventure (once he steps out of his pure - blood supremacist mindset) appealing, and Draco could be impressed by Hermione's vast knowledge.
Well, one possibility is that Ash wakes up with all the powers of Mew. He could then use these powers to quickly become the best Pokémon trainer ever. He might be able to communicate telepathically with all Pokémon, understand their needs and desires instantly, and use his newfound psychic abilities to battle more effectively. For example, he could levitate his Pokémon during battles, giving them an unexpected advantage.
Another way could be through a potion gone wrong. In the wizarding world, potions can have all sorts of strange effects. If Draco was trying to brew a potion for some other purpose, like trying to gain an advantage in a competition or something, and made a mistake in the ingredients or the brewing process, it could result in him being transformed into Hermione. This would force him to experience life from her perspective, which could be a huge character - development opportunity for him.
One possibility is a spell gone wrong. Maybe they were both involved in some sort of experimental magic in the library or an abandoned classroom, and the spell had unforeseen consequences on their physical states.
In the 'beerus wakes up early fanfic', maybe Beerus wakes up early and decides to go on a sudden hunt for the best food in the universe. He could startle the local inhabitants of the planets he visits as he demands the most delicious delicacies.
Well, perhaps they could start the day with a friendly chat over breakfast. Harry might share some of his wizarding world stories with Gabrielle, and she could be really fascinated by them.
She may wake up in a situation where she is being hunted by vampires or other dark forces. It's possible that she meets a friendly vampire or witch who helps her understand the strange world she's now a part of. Or perhaps she stumbles upon a long - lost mystery that is crucial to the survival of the town and its inhabitants.