Themes of destiny and free will also play a big part. In both shows, the characters struggle with their fates. In a crossover, they can compare notes on how they've dealt with prophecies and whether they believe they can change their destinies. For instance, Merlin was prophesied to bring about great change, and the Winchesters have often been part of prophecies in their own world.
One great Merlin - Supernatural crossover fanfic is 'The Hunter and the Warlock'. It combines the magic of Merlin's world with the demon - hunting aspect of Supernatural in a really interesting way. The characters from both shows interact in a believable manner, making for an exciting read.
Often, such crossovers might have Merlin using his magic to assist the Doctor in facing interdimensional threats. Or, the Doctor could travel to Camelot and encounter Merlin's challenges. There could also be time paradoxes and alternate timelines explored.
One more key element is the conflict. It could be a conflict between the two groups at first, like misunderstandings due to their different ways of operating. Or it could be an external threat that forces them to work together. This conflict drives the story forward and makes the reader interested in seeing how the characters from these two different shows will resolve their differences and work towards a common goal.
Key elements could be crossovers of their iconic tools or weapons, like the Doctor's sonic screwdriver and the Winchesters' angel blades. Plotlines could see them dealing with a threat that spans multiple timelines and realities. Or it could be a story where a character from one show accidentally alters the timeline of the other, leading to all sorts of complications.
One key element is the alteration of the original storyline. This could involve changing the order of events, like Merlin revealing his magic earlier. Another is character development. For example, giving Morgana a different path that doesn't lead her to become evil. And of course, the exploration of magic in new ways, such as Merlin having a different type of magic or learning it from a new source.
Teamwork is also a major theme. Since the characters from both universes have different skills and knowledge, they have to learn to work together. In many fanfics, we see Merlin teaching the Winchesters about his kind of magic, and in return, the Winchesters showing Merlin how to deal with the more modern - day and dangerous supernatural entities. This theme of cooperation is what makes the crossovers so interesting.
Character exploration. Hannah is an angel, so her angelic nature, powers, and how she uses them in different situations are often explored. For example, how she heals or uses her grace.
One key element is the relationship dynamic. Since Merlin has magic and Arthur is the king - to - be, there's a power play and a sense of mutual respect that develops. Another is the setting. The medieval - like world of Camelot adds a lot of charm and context to their relationship. And of course, the magic itself is a big part of it as Merlin uses her powers in unique ways in relation to Arthur.
In a Naruto and Sekirei crossover fanfiction, one key element could be the power systems. Naruto has its chakra - based ninjutsu, while Sekirei has the unique Sekirei powers. Combining these would be really interesting. For example, a Sekirei character might learn to use chakra to enhance their existing powers or a Naruto character could gain some Sekirei - like abilities. Another element is the characters' personalities. The determined and often brash Naruto characters mixing with the more diverse and complex Sekirei personalities can lead to great character interactions and storylines. There could also be a new world - building aspect, like how the two very different worlds of Naruto and Sekirei merge or co - exist.
Character interaction is really important. If Zoro is in a crossover with, say, other anime characters, how he interacts with them, whether they become friends or foes. Also, the plot often revolves around a new threat or goal. Maybe Zoro has to help save a world that's not his own from destruction. And the setting plays a big role too. A strange new world can bring out different aspects of Zoro's personality.