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.
Love and romance also play a role. Sometimes it's a forbidden love, like loving someone from a different social class or a different magical group. In other cases, it's a slow - burning love that develops over the course of the story as the female character grows and changes. For instance, she might fall in love with a character who initially was her enemy but then they see each other in a new light.
I really like 'A Female Merlin's Quest'. In this fanfiction, the female Merlin is portrayed as a strong and independent character. She uses her magic to help Arthur in different ways, and their bond is both platonic and filled with mutual respect. The author does a great job of creating a believable world within the Arthurian setting.
One big theme is the exploration of magic. In many Merlin female fanfictions, the female characters are either learning magic, or have a special connection to magic that is different from the male characters. For instance, a female character might have a more intuitive approach to magic. Also, the theme of friendship is present. A female character could form a strong friendship with Merlin or other characters like Guinevere, and this friendship is crucial in their adventures.
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.
There are quite a few Merlin female - centered fanfiction stories out there. You can start by looking on fanfiction platforms like Archive of Our Own. Some might focus on the relationships between Merlin and female characters, exploring different plotlines and character developments.