There could be. Fanfiction is a world of endless possibilities. Given the popularity of the characters Merlin and Meliodas, it wouldn't be surprising if someone has written a fanfiction with this concept. To find it, you might also want to check out some fan communities dedicated to the series. They often have discussions about unique fanfictions, and someone might be able to point you in the right direction if such a fanfiction exists.
In fanfiction, Ygraine is often a crucial figure. As the mother of Arthur and Merlin's connection to Arthur's birth, she can be a symbol of destiny. Her role might be expanded to show how her actions or even her existence influenced the fates of both Merlin and Arthur. For example, some fanfictions may explore how her death was not just an event but a catalyst for future events in their lives.
One possible plot could be Merlin hiding her true identity from Meliodas as he grows up. She could be watching over him from afar, secretly guiding his actions and protecting him from afar while he is on his adventures.
Often in fanfiction, Ygraine is a complex character. She may be portrayed as a victim of Uther's desires in one aspect, but also as a strong - willed woman. When it comes to her relationship with Merlin and Arthur, she could be the one who unknowingly weaves their fates together. For instance, her death could be the starting point of a chain of events that bring Merlin and Arthur closer, and fanfiction writers can play with this concept in many different ways.
Well, it changes a great deal. Traditionally, Merlin's origin is somewhat mysterious but not related to Loki. If Loki is his mother in this fanfic, it might introduce elements of Norse mythology into the Arthurian legend. For example, Loki's trickster nature could be passed on to Merlin, making him even more cunning and unpredictable in his magic and interactions. It could also mean that Merlin has a connection to the Norse gods, which might lead to different plotlines like dealing with Norse threats or having allies from that pantheon.
Archive of Our Own (AO3) is a great platform for Merlin mother fanfictions. It has a large collection of works that cover various aspects of the Merlin universe, including those focused on Merlin's mother. There are different tags and filters you can use to easily find the fanfictions you're interested in.
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.