The 'mentor - mentee' trope is often seen in these fanfictions. Maybe Dumbledore mentoring one of the Pevensie children or Aslan guiding Harry Potter. And the 'power - up' trope, where a character discovers new and greater powers. For instance, a Narnian character getting magic powers similar to those in the wizarding world, or a Hogwarts student gaining the ability to communicate with Narnian animals. These tropes add new and exciting dimensions to the already rich worlds of Chronicles of Narnia and Harry Potter.
One popular trope is the 'what - if' scenario. For example, what if Harry Potter ended up in Narnia? Or what if the Pevensie children went to Hogwarts? Another common one is the 'alternate universe' trope, where the entire story is set in a different world with different rules but still features the beloved characters. There's also the 'character - centric' trope, like a story that focuses solely on Hermione's adventures in Narnia or Edmund's time at Hogwarts.
Harry Potter himself is often a popular choice to cross over. His courage and magical abilities make him a great addition to the Narnian world. Also, Aslan from Chronicles of Narnia is sometimes brought into the Harry Potter universe. His powerful and wise nature can bring a new perspective to the wizarding world.
One common theme is the exploration of different magic systems. Since both series have their own unique magic, fanfictions often compare and combine them. Another theme is friendship. Just like in the original works, characters from both universes form bonds in the fanfictions. Also, the battle between good and evil is a recurring theme, as it is central to both 'Harry Potter' and 'Chronicles of Narnia'.
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.
Hermione and Lucy are often popular to pair. Hermione's intelligence and knowledge of magic could complement Lucy's kind - hearted and brave nature. They could go on adventures together, with Hermione using her logical thinking and Lucy's intuition to solve problems.
One interesting crossover could be the characters from Hogwarts finding a magical portal to Narnia. They might be amazed by the talking animals and the different magical rules there. For example, Harry could use his magic skills to help the Narnians in a battle against the White Witch.
One popular trope is the 'Older Brother Figure'. Bill often takes on this role for Harry in fanfiction. For example, in many stories he gives Harry advice on dealing with Voldemort. Another is the 'Mysterious Curse - Breaker' trope. Since Bill is a curse - breaker, fics often play up the mystery and danger of his job. A third popular trope is 'Weasley Family Bonds'. Fanfictions explore how Bill's actions and decisions affect the whole Weasley family.