It's also possible that Harry wakes up married to a Muggle in fanfiction. This would create a whole new set of challenges as he has to deal with the wizarding world and his new Muggle spouse. He would have to explain the magical world to them and figure out how to balance both parts of his life.
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.
Another possibility is that Harry ends up married to a Muggle he meets in Vegas. This would be really interesting as he has to navigate the differences between the wizarding world and the Muggle world while being married to someone who has no idea about magic at first. It would be a story of discovery and adjustment for both Harry and his new spouse.
One popular 'Harry wakes up married' fanfiction might involve him waking up married to Hermione. In this scenario, they could have been under the influence of a magical spell that made them get married without realizing it. Then they have to deal with the consequences, like telling their friends and trying to figure out if they actually have feelings for each other.
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.
Some pure - blood supremacist families who don't approve of their marriage could be antagonists. They might try to cause trouble for Harry and Luna, maybe by spreading false rumors or trying to undermine their work in the wizarding world.
Perhaps there are challenges from their friends and the wizarding community. Some might be shocked or disapproving of their union. This could lead to them having to defend their relationship and face various social pressures. Maybe they have to work together to prove that their marriage can work despite the unorthodox start.
Hermione would likely be a key friend. She might be initially very confused and try to analyze the situation rationally. She could be worried about how this will affect Harry and his future, and might try to find a way to reverse the situation if it seems harmful to Harry.
It could be Draco Malfoy. In some fanfictions, the dynamic between a female Harry Potter and Draco is reimagined, and they might end up secretly married as a way to show a new kind of relationship between the two formerly antagonistic characters.
Well, Harry could marry one of the veelas. Given their beauty and allure, it would add an interesting dynamic to the story. Their relationship could involve Harry having to deal with the jealousy of other wizards and also the cultural differences of the veelas. Or he could marry a descendant of one of the founders of Hogwarts. This would tie in a lot of Hogwarts history into their relationship, and their wedding could be a grand event that unites different factions within the wizarding community who have ties to the founders.
Some fanfictions might even pair Harry with a new character. For example, a foreign witch who comes to Hogwarts for an exchange program. She could bring a new cultural element to the story and her relationship with Harry could be based on their shared love for magic and exploration of different magical cultures.
It could be a random stranger he meets at a Vegas nightclub. Maybe she's a singer or a dancer there.