Yes, in some fanfictions, Merlin might be depicted as having a tattoo. It could be used as a plot device, perhaps to symbolize something important about his character, like his hidden power or a connection to a particular group or magic.
The tattoo might be a sign of his destiny. In some fanfictions, Merlin is on a grand quest, and the tattoo could be a map or a clue that leads him to fulfill that destiny. It could also be a mark of a pact he made, either with a magical being or for the greater good of Camelot.
The Druid tattoo on Merlin in fanfiction could have multiple meanings. Maybe it was given to him by a Druid elder as a symbol of protection. It could be a key to unlocking certain magical abilities that are related to the Druids. In some fanfictions, it might be a way to show that Merlin has a dual identity - his normal self and his Druid - influenced self, and the tattoo is a physical manifestation of that.
The tattoo can be a source of mystery. Other characters may be curious about it, leading to investigations and interactions that further the plot. For example, Arthur could be suspicious at first but then learn more about Merlin's Druid side through the tattoo. Also, it could be a factor in Merlin's search for his true identity or purpose within the fanfiction's world.
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.
The tattoo might be related to his family. In fanfiction, it could be a family crest or a symbol that has been passed down in his family for generations. It might be something that he got as a way to connect with his roots, especially if he has a complex family history in the story.