Well, without having read the 'liesl and merlin novel', it might be a story that delves into the relationship between Liesl and Merlin. Maybe it's set in a fantastical world where Merlin has his magical powers and Liesl has some unique qualities of her own. They could be on a quest to save their world from some dark force, or they might be exploring the secrets of magic and their own identities within the story.
One great female Merlin fanfic could be one where she uses her magic in unique ways to save Camelot. For example, in a story she might use her powers to heal the knights secretly when they are injured in battle. This would add an interesting twist as she has to hide her actions from Uther.
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.
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.
One possible reason could be a magical curse. In many Merlin fanfictions, magic is often used as a plot device. A dark sorcerer might cast a spell on Merlin to make him forget certain things, perhaps to prevent him from using his powerful magic to oppose them. Another reason could be a self - imposed forgetting due to trauma. Merlin has witnessed and endured a great deal of pain and suffering. To protect his own mental state, his mind might subconsciously block out certain memories.
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.