Well, the theme of change is significant. His discovery of immortality changes his view of himself, his relationships, and his future. And there's also the theme of power. His immortality can be seen as a form of power, and he has to decide how to use it, whether for good or bad. This power can also attract enemies who want to either control him or take his immortality away.
A major theme is the responsibility that comes with immortality. Merlin often has to use his new - found immortality to protect Camelot or the people he cares about in different ways. Also, the theme of secrecy is quite common. He may need to keep his immortality a secret to avoid being hunted or feared by others.
I recall a fanfiction named 'Merlin's Immortal Fate'. In it, Merlin discovers he's immortal when he survives a fatal wound that should have killed him. This discovery changes his perspective on life and his role in Camelot. He starts to use his immortality to protect the kingdom in ways he hadn't thought of before. There's also a fanfic where his immortality is tied to a magical spell cast long ago, but I can't remember the exact name right now.
The theme of friendship is also quite common. Despite their age difference, Naruto and Tsunade can form a unique friendship. In fanfictions, they might share their past experiences, their hopes and dreams. Tsunade might open up to Naruto about her losses, like the death of her loved ones, and Naruto can comfort her with his positive outlook on life. This friendship can then be a driving force in the story, helping them face various challenges together.
One common theme could be the struggle between her magical duties and the pregnancy. Since Merlin has important magical tasks, being pregnant might create a conflict.
A common theme is the conflict between loyalty and personal beliefs. Merlin has always been loyal to Arthur, but his loyalty to magic and his own kind might come into conflict. This internal struggle is often explored in these fanfictions.
One popular plot is that Arthur is initially shocked and angry when he discovers Merlin's magic, but then he gradually realizes how Merlin has been using it to protect him all along. For example, in some fanfictions, Merlin uses his magic to save Arthur from a seemingly impossible battle, and Arthur finally comes to terms with Merlin's powers and they become even closer friends, united in a new way to protect Camelot.
In this fanfiction, perhaps Merlin discovers that his pregnancy is part of some ancient prophecy. He could then be on a quest to understand the true meaning of it all while dealing with the normal pregnancy woes. He might also face discrimination or disbelief from some in Camelot who can't fathom a man being pregnant, which would add to the drama.
Maybe it's a mysterious figure from Merlin's past. Perhaps he had an encounter with a powerful magical being, and some kind of spell or pact led to the pregnancy. It could be a long - forgotten event that now resurfaces with this unexpected pregnancy.
Some fanfictions show Arthur becoming more dependent on Merlin's magic in certain situations. For instance, when facing a powerful enemy that can't be defeated by normal means. This changes the dynamic from just a king - servant relationship to more of a partnership. Merlin, on the other hand, might feel more confident in using his magic openly around Arthur, which also alters their interactions. They could have more equal conversations and decision - making processes.
One common theme could be loyalty. Merlin, being a soldier, would show loyalty to his comrades, his cause, or his kingdom. Another theme might be sacrifice. Soldiers often have to make sacrifices, and in a Merlin - as - a - soldier fanfiction, he might sacrifice his personal desires for the greater good. Also, the theme of honor could be prominent. He would strive to uphold his honor in battle and in his interactions with others.
One common theme is identity crisis. Merlin has to come to terms with being the son of a man who has persecuted his kind (magic users). Another theme is family secrets. The fact that this truth has been hidden for so long creates a lot of drama.