A sense of adventure would be in Merlin's story. He has been on many quests and journeys. He might talk about traveling to far - off lands in search of a solution to a problem in Camelot. He could describe the different landscapes he saw, from snow - covered mountains to vast deserts. And of course, there would be elements of good versus evil, as Merlin constantly battles against the forces of darkness to protect the light in Camelot.
If Merlin died in the fanfiction, it could trigger a series of tragic events. The balance of power could tip, and the villains might seize the opportunity to attack. It could also bring out hidden strengths in the remaining characters as they try to cope with the loss.
He would become more brooding. As a vampire, he'd have to deal with new emotions like the constant thirst for blood. This would make him more withdrawn from his friends in Camelot at first.
He would probably interact with Dumbledore a lot. They are both very powerful wizards and could share a great deal of magical knowledge.
Merlin could bring his magic to the Avengers. His knowledge of ancient spells and potions might help in battles against powerful foes. For example, he could use magic to disarm enemies or create protective shields.
If Merlin is a Dark Lord in fanfiction, it could be an interesting exploration of the 'nature vs. nurture' concept. Maybe he turned dark because of the constant fear of being discovered and persecuted for his magic. His actions as a Dark Lord might include taking over Camelot or exacting revenge on those who wronged him in the past. It gives a whole new perspective on his character.
In Merlin fanfiction, Merlin might tell a story about his magic origins. Maybe he shares how he discovered his powers as a young boy in Ealdor. He could talk about the first time he made something extraordinary happen, like making a flower bloom in the dead of winter. This story would not only be about his powers but also about the fear and wonder he felt at that moment.
It could be for comedic effect to lighten the mood and add some fun to the story.
In a more humorous take on the 'Merlin tied to a bed' fanfiction, it could be that Merlin has fallen ill and the overly - protective Gaius has tied him to the bed to make sure he rests. Merlin, being the active person he is, complains constantly. Arthur comes to visit and teases Merlin about finally being still for once. Meanwhile, Merlin tries to convince Arthur to let him loose, promising that he'll be fine. Eventually, Gaius relents and unties Merlin, who then immediately starts bustling around the room.
Merlin is a very interesting character. His magical powers and his role as a servant with a big secret in the original story make him a prime target for fanfiction. People like to write about him because they can explore his untapped potential in their stories.
A 'Naruto is Merlin' fanfiction could have Naruto in the Arthurian world, using his ninja knowledge to survive. He might teach the knights some of his combat moves. Or, if it's set in the Naruto world, he could be the only one with Merlin - like magic, causing others to be both in awe and a bit scared of him. He could use magic to heal wounds like in the Naruto universe where medical ninjas exist, but with a magical twist.