One interesting 'Harry Potter drunk fanfic' could be about the trio - Harry, Ron, and Hermione - getting accidentally drunk during a celebration at Hogwarts. Maybe they start acting really silly, like Ron trying to teach the portraits how to do a drunken dance. It could be a humorous take on their characters, showing a side of them we don't usually see in the original books.
There could be a 'Harry Potter drunk fanfic' where Draco Malfoy gets drunk at a Slytherin gathering. He might end up saying things he doesn't really mean, like admitting that he's actually a bit envious of Harry's popularity. This could lead to some unexpected character development and interactions with other Slytherins or even with the Gryffindors.
There could be a fanfic where the whole Gryffindor team gets drunk after a big Quidditch win. They start flying around the pitch in a rather wobbly manner. Harry, being the team captain, tries to keep everyone in line but ends up joining in the fun. This fanfic could explore the lighter side of the Hogwarts students and how they let loose in a magical, albeit drunken, way.
Some fanfictions in this genre might explore how the wizarding laws regarding drunk marriages work. For example, in a story, Hermione could be researching the ancient laws about such marriages after Ron and another character drunkenly exchange vows. She discovers loopholes or strict regulations that determine whether the marriage can be easily annulled or if they have to go through a complex process, all while the relationship between the two 'drunk - married' characters develops in unexpected ways.
There are 'Harry Potter fanfic fanfic' that focus on time - travel scenarios. Imagine if a character from the later books travels back in time to the first year at Hogwarts. This can create a lot of interesting plot twists as they try to change or preserve the original story line. Also, some fanfic fanfic might center around magical creatures in a way that wasn't fully explored in the original books, like the life of a hippogriff in more detail.
Some 'Harry Potter and Lily Potter fanfic' might focus on alternate universes. In one such story, Lily could have survived Voldemort's attack. She would then have to raise Harry in a world that is still recovering from the Dark Lord's first rise. This would change a lot of the dynamics in the story. Harry would grow up with his mother's love and protection, and Lily would have to deal with the knowledge that Voldemort could return at any time.
Yes, one popular theme is characters getting drunk and accidentally revealing secrets. For example, Hermione might get drunk and blurt out her feelings for Ron that she's been hiding. It adds a bit of drama and comedy to the story.
I'm not really in favor of the 'slaves' concept in fanfic as it can be a bit disturbing and against the positive spirit of the original 'Harry Potter' series. However, some fanfics might explore power dynamics in a more metaphorical way. But overall, the official 'Harry Potter' works focus on friendship, courage and magic, not slavery themes.
One interesting 'Harry Potter breed fanfic' story could be about the breeding of Hippogriffs in a more domesticated setting. Maybe it explores how wizards are trying to train them for various tasks like mail delivery in a different way than using owls.
One interesting 'Harry Potter Siren Fanfic' could be about a siren who uses her charm to influence the events at Hogwarts in a subtle way. Maybe she befriends Harry and tries to guide him through some challenges using her siren - like powers, which are different from magic but still effective.
There might be a fanfic where a MACUSA auror teams up with a Hogwarts student. They could be working together to stop a dark wizard who is causing chaos in both the American and British wizarding worlds. The story could explore how their different magical educations and approaches to magic come into play.
One interesting 'Harry Potter glutton fanfic' could be about Harry discovering a magical kitchen that creates endless amounts of his favorite wizarding treats. He goes on a bit of a food - filled adventure, trying all sorts of new and old magical delicacies.