The 'alternate universe' trope is also quite popular. In this, the two universes are merged in some way from the start. Maybe Hogwarts is located near Downton Abbey and students interact with the local aristocracy. Or perhaps the wizarding world has always been a part of the Downton Abbey world, but hidden, and now things are starting to come to light. This trope allows for a lot of creative storytelling and exploring how the two very different settings can co - exist.
Well, one interesting plot idea is the introduction of magic into the Downton Abbey society. Imagine the Dowager Countess's reaction to seeing a wizard perform magic right in front of her! Another idea could be a Hogwarts student getting lost and ending up at Downton Abbey, and having to find their way back to the wizarding world while dealing with the strict social norms of Downton.
I think Sirius Black and the Dowager Countess would be a hilarious pair. Sirius is a bit of a rebel, always defying the norms of the wizarding world. The Dowager Countess is set in her ways and very proper. Their interactions would be full of comical misunderstandings. Sirius might accidentally use magic in front of her, and she would be both shocked and intrigued, trying to maintain her dignity while dealing with this strange new phenomenon.
One idea could be a crossover where the magic of Harry Potter enters the refined world of Downton Abbey. For example, a wizard or witch could be disguised as a servant at Downton and secretly use magic to solve various problems in the household, like mending broken heirlooms with a spell or helping a character with a love problem using a love potion in a very discreet way.
Some Tumblr blogs are also dedicated to sharing and discussing such fanfictions. Just search for relevant tags like #DowntonAbbey #HarryPotter #Fanfiction. However, the quality can vary widely, so it might take some time to sift through. Another option is to join relevant fanfiction groups on Facebook or Reddit. Members often share their favorite stories or even write their own and post them within the group for others to enjoy.
Well, a common theme is romance. Fans often write fanfiction to give their favorite characters different romantic pairings or to expand on the existing romances. There's also the theme of social change. Since Downton Abbey is set during a time of great social upheaval, fanfiction might focus on how the characters adapt to new social norms. And then there's the theme of family secrets. Uncovering hidden aspects of the Crawley family history or the secrets among the servants can make for really interesting fanfiction.
In 20 years of Harry Potter fanfiction, there are several popular tropes. The 'Powerful!Harry' trope is quite common. Fans like to write about Harry having even more power than in the books. Also, the 'Soulmate' trope is popular, especially when it comes to romantic pairings. And the 'Dark!Hermione' trope has emerged, where Hermione takes a more morally ambiguous or even dark path, which is an interesting twist on her character.
The 'Power Transfer' trope is often seen. This is when a character from one series gains the power or ability from the other series. Say, Harry getting the ability to see ghosts like Sam does in Supernatural. And then there's the 'Shared Enemy' trope. A threat that is dangerous to both the wizarding world and the Supernatural world forces the characters to cooperate, creating exciting storylines.
A very common trope is the 'Dark!Harry' concept. In these stories, Harry turns to the dark side, either because of being mistreated or for some other reason. It's interesting to see how the story unfolds when the main hero becomes an anti - hero. Also, the 'Teacher - Student Romance' trope exists, though it's a bit controversial. But it does add an element of forbidden love to the stories.
One popular trope is the 'alternate universe' where the story takes place in a different setting or with different rules from the original series. For example, a wizarding world where Voldemort never existed. Another is the 'character redemption' trope, like Draco Malfoy getting a chance to be a good person from the start. And 'romantic pairings' that fans love to explore, such as Hermione with someone other than Ron.
One common trope is the 'fish out of water' situation where Castiel is suddenly in the Harry Potter world and has to figure out how things work. Another is the 'powerful ally' trope where he becomes a crucial help to Harry and his friends.