One common theme is friendship. Merlin and Arthur's deep friendship is often explored further, showing how they support each other through tough times. Another theme is sacrifice. Since they both have important roles, they often have to make sacrifices for the greater good, which is a big part of many fanfics.
The theme of self - discovery is prevalent too. As they are thrown into new worlds or situations, they learn more about themselves. Arthur might discover new leadership qualities or Merlin might find new aspects of his magic. These themes are often intertwined, creating complex and engaging stories in merthur crossover fanfiction.
A common theme is the secrecy and the public perception. Since their relationship might be frowned upon in the Camelot setting, they often have to keep it a secret at first. This creates a lot of tension and drama. Also, the theme of mutual support is prevalent. Merlin supports Arthur in his kingly duties, and Arthur supports Merlin in dealing with the challenges of being a magic - user in a non - magic - friendly place.
Love and passion are very common themes. Since it's a 'lemon' fanfic, there's often a focus on the physical aspect of their relationship, but also the deep emotional connection between Merlin and Arthur.
A common theme is the overcoming of obstacles. Since their relationship has many barriers in the show, like Arthur's initial ignorance of Merlin's magic and the social differences between them, fanfiction often focuses on how they break through these. Also, there's the theme of adventure. Writers might send them on new quests together, building their bond in the process.
One common theme is friendship. Since Merlin and Arthur have such a complex relationship in the original series, fanfictions often explore different aspects of their friendship, like loyalty and sacrifice.
One interesting Merthur time travel fanfiction could be 'Back to the Beginning'. It takes Merlin back in time to the day he first arrived in Camelot and he has to navigate his relationship with Arthur all over again, but this time with the knowledge he has from the future. Another great one might be 'Time's Twist' where Arthur travels to the future and sees how Merlin has been waiting for him for centuries. There's also 'Journey Through Time' which has both of them traveling through different eras of Camelot's history, changing things and facing the consequences.
There's also 'Merlin's Secret'. This fanfic focuses on Merlin having to keep his magic a secret from Arthur in a more complex way than in the original show. It has a lot of tension and the relationship between the two is portrayed in a very interesting and heart - warming manner.
I really like 'Merthur's A/B/O Destiny'. It delves deep into how the A/B/O elements affect their world and their connection. It has some really interesting plot twists that add to the overall charm of the fic. For example, the way Merlin's omega nature is intertwined with his magic in this story is quite creative.