Another theme is the idea of using one's unique abilities for the greater good. Merlin has his magic, and Nobel has his scientific knowledge. In many fanfictions, they come together to solve problems that neither could solve alone, like saving a kingdom from a mysterious plague or averting a natural disaster.
One common theme is the clash and eventual harmony between magic and science. Since Merlin represents magic and Nobel represents science, their interactions often explore how these two different forces can coexist.
One common theme is the shock and disbelief among the characters. Given Merlin is male in the original series, his pregnancy is quite unexpected, so characters often struggle to come to terms with it.
Redemption is also a common theme. Since Morgana often takes a darker path in the original, many fanfictions explore the possibility of her redemption, with Merlin being a key factor in that journey. Love, in various forms, is another theme. It could be a platonic love born out of mutual understanding or a more romantic love that defies the odds.
There are several common themes. Empathy is one. Characters in the fanfiction, especially Arthur, often have to learn to be more empathetic towards Merlin. Another theme is adaptation. Merlin has to adapt to his new situation, and this can range from how he uses his magic to how he moves around Camelot. Some fanfictions also explore the theme of isolation, as Merlin might initially feel alone with his blindness, but then overcome it through his relationships.
Love and friendship are also common themes. In many Merlin reincarnation fanfictions, he forms new relationships. These relationships can be romantic or platonic. For example, he might meet someone who reminds him of an old friend from his past life. And of course, the theme of redemption can also be present. If he made mistakes in his past life, he might get a chance to make up for them in his new reincarnation.
Another theme is friendship or an unlikely alliance. Merlin and the Lamia usually form a bond that is unexpected, and the fanfictions often revolve around how they support each other despite their differences. For example, in some stories, the Lamia may have powers that Merlin needs to defeat a common enemy. They then have to learn to trust each other.
One common theme is magic. Since both Merlin and unicorns are associated with magic, their interactions often explore different aspects of magic, like how it can be used for good or how it can be misused.
One common theme is the element of secrecy. Since Merlin is often in a situation where his pregnancy is something that could cause a stir in Camelot, he has to keep it hidden. This secrecy adds a lot of drama and tension to the story.
Power and responsibility is a very common theme. As a dragonlord, Merlin has great power, and fanfictions often explore how he deals with the responsibility that comes with it.
Friendship is a very common theme. Since they are both in Camelot, their bond as friends is often explored. Another common theme is loyalty. Percival is known for his loyalty, and this is often emphasized in fanfiction in relation to his relationship with Merlin.
The exploration of identity is also common. Merlin may question who he is while being a slave. Is he still the same person with the same destiny? How does being a slave change his view of himself and his role in Camelot? These are the kinds of questions that are often explored in such fanfictions.