One characteristic could be that he has a more modern personality. In fanfiction, writers might give him contemporary values or speech patterns.
In fanfiction, Merlin might have enhanced powers. Since fan writers have creative freedom, they could make his magic more powerful or give him new abilities that he didn't have in the original stories. Also, his relationships could be different. For example, he could have a romantic relationship with a character that was not part of his story in the original lore.
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.
It could be for comedic effect to lighten the mood and add some fun to the story.
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.
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 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.
In fanfiction, a strong Merlin might be physically powerful. He could have enhanced magical abilities that allow him to defeat powerful enemies with ease. For example, he might be able to cast spells that no one else can.
One characteristic is the use of comical situations. For example, Merlin getting into trouble because of his magic in a light - hearted way, like accidentally making Arthur's hair grow really long. Another is humorous dialogue. The way Merlin and the other characters interact with witty banter makes it funny.
Maybe it could be considered so because of the magic theme. Merlin is well - known for his magic, and in 'Harry Potter', magic is everywhere. The idea of a powerful wizard guiding the fate of others, like Dumbledore in 'Harry Potter' could be seen as similar to Merlin's role in his stories. But this is a very loose connection.
Another possible candidate in fanfictions could be Neville Longbottom. He has a deep connection to the magical world and shows great courage and growth throughout the series. His latent magical abilities, which are finally fully realized, could be interpreted as him being an heir of Merlin in a way. He has the potential to be a great wizard, just like Merlin was.
He could become a Dark Lord due to betrayal. If someone he trusted deeply, like Arthur perhaps, turned on him because of his magic, it might push him over the edge.