One of the main themes is the conflict between Merlin's secret magic and the public perception in the royal court. There's also the theme of destiny. Merlin is often seen as having a great destiny in relation to the royalty. And love, whether it be platonic or romantic between Merlin and the royal characters, is a frequent theme. For example, in many fanfictions, Merlin forms a deep bond of friendship with Arthur that is almost like a brotherly love. His loyalty to Arthur and the kingdom is constantly tested by his need to hide his magic.
In Royal Fem Merlin fanfiction, Merlin is female, which changes the power dynamics and relationships. For example, her interactions with Arthur might have a different romantic or platonic undertone. Traditional fanfiction usually follows the male Merlin's story.
One great Royal Fem Merlin fanfiction could be 'The Queen's Enchantress'. It reimagines Merlin as a female in a royal court setting with a lot of political intrigue and magic. Another is 'Fem Merlin's Ascent' which focuses on her rise to power while dealing with the challenges of being a female with magic in a male - dominated world. There's also 'Royal Fem Merlin: The Secret Alliance' that has her forming unexpected alliances within the kingdom.
One great Royal Merlin fanfiction is 'The King's Enchanter'. It beautifully weaves the relationship between Merlin and the royalty. Another is 'Merlin's Royal Destiny' which takes an interesting twist on Merlin's role in the royal court. And 'Royal Secrets and Merlin's Magic' is also a good read, full of mystery and magic.
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.
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.
One more idea is that Merlin is tied to the bed in a dream - like state created by an ancient magic. In this dream world, he is faced with his deepest fears and insecurities. His friends in the real world notice his strange behavior and try to wake him up. Merlin, while in the dream, has to find the source of the magic that is binding him and break free. Through his inner strength and the memories of his friends, he manages to do so and wakes up, having learned more about himself and his magic.
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.
He could become a Dark Lord due to betrayal. If someone he trusted deeply, like Arthur perhaps, turned on him because of his magic, it might push him over the edge.