First, focus on the character of Merlin. Since it's a 'tiny problem', it should be something that doesn't overshadow his main role but still adds depth. For example, you could start with Merlin waking up one day and realizing he has lost his ability to communicate with his familiar, which is a small but significant part of his magic.
Obviously, Merlin himself. And Arthur could be in it too. Maybe Arthur doesn't initially recognize Tiny Merlin and there are some comical misunderstandings between them.
Perhaps a story where Tiny Merlin accidentally shrinks the whole Camelot and has to figure out how to reverse the spell. He would have to use his wits and small - size advantages to solve the problem. This could involve sneaking into small spaces in the shrunken Camelot to find the source of the spell. It would be a really fun and adventurous story.
It could be about a minor issue that Merlin faces in Camelot. Maybe it's a small spell - casting mishap that causes some comical chaos in the court. For example, he accidentally turns Arthur's hair pink while trying to do a simple cleaning spell on his chambers.
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.
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.