One popular trope is the 'fish out of water' situation. For example, when a character from Harry Potter is suddenly thrust into the world of Code Geass or vice versa. They have to learn new rules and ways of life. Another is the power - up trope. A character might gain new abilities by combining magic from Harry Potter and the Geass power. For example, a wizard could use a spell to enhance the effectiveness of their Geass.
Well, a very common trope is the culture clash. The magical society in Harry Potter has its own set of values, traditions, and hierarchies, while Code Geass has a very different political and social structure. Fanfictions often explore how characters from both universes deal with this clash. Also, the 'enemies to allies' trope is quite popular. Characters who start off as foes due to their different backgrounds end up working together. This can lead to some really interesting character development and plot twists.
Lelouch is quite popular. His intelligence and strategic mind could be very interesting in the Harry Potter world. He could use his skills to outwit Voldemort or manipulate the Ministry of Magic. Another popular one is Suzaku. His sense of justice could be explored in the context of Hogwarts' houses and their values.
There's also the 'soulmates' trope. Some fanfictions depict Remus and Sirius as soulmates, with a special connection that goes beyond the normal. This can be a magical bond or just a really deep emotional connection. And then there's the 'reunion' trope. After being separated, for example, when Sirius was in Azkaban, fanfictions often focus on their emotional reunion and how they pick up their relationship from there.
Yes, there are quite a few. Some fanfictions explore how the magic from Harry Potter's world could interact with the political and strategic elements in Code Geass. For example, there might be stories where characters use spells to gain an edge in the mecha battles of Code Geass. However, the quality can vary greatly. You can find them on various fanfiction websites by simply searching for 'Harry Potter x Code Geass' in the search bar.
One interesting plot twist could be that the Death Eaters in the Harry Potter world form an alliance with the Britannian Empire from Code Geass. This would create a powerful and evil force that the protagonists from both worlds would have to fight against. Another could be that a character like Lelouch, with his Geass power, gets transported to Hogwarts and uses his wits and power to outmaneuver Voldemort. Maybe he could even use his Geass on some of Voldemort's followers to turn them against him.
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.
A crossover might also involve the characters' moral compasses. The complex moral choices in Code Geass, like Lelouch's decision - making, could be juxtaposed with the good - vs - evil battles in Harry Potter. Harry, who is often seen as a paragon of good, could be influenced by Lelouch's more morally ambiguous actions, leading to some very interesting character development.