It could also be that Gwen has been influenced by some external force or another character. For example, an enemy of Merlin might have convinced Gwen that Arthur would be better off without Merlin. They might have told her lies about Merlin, and as a result, she takes the step of making Arthur forget him.
One possible motive could be jealousy. Maybe she sees Merlin as taking too much of Arthur's attention and time, and she wants Arthur all to herself. So she uses some sort of magic or spell to make Arthur forget Merlin.
The complex relationships. Merlin's loyalty to Arthur, his friendship with Gwen, and the love triangle between Arthur, Gwen and the unspoken feelings Merlin might have. It's all very interesting to explore in fanfiction.
In a fanfiction, it could be that an evil sorcerer casts a spell on Gwen and Arthur's unborn baby. The baby starts to show strange and dangerous symptoms. Merlin is the only one who can sense what's really wrong. He has to race against time to break the spell. He delves deep into his magic knowledge and also consults ancient magical texts. Along the way, he has to hide his actions from those in Camelot who still distrust magic, all while trying to save the baby and ensure the future of Camelot's royal family.
One great fanfiction is 'The Once and Future King's Love'. It beautifully weaves the relationships between Merlin, Gwen, and Arthur. It shows their deep friendship and the budding romances in a really engaging way. The author does a wonderful job of staying true to the characters' personalities from the show.
I'm not entirely sure if there is a specific one. While fanfiction can cover a vast range of scenarios, including some that deviate from the expected character relationships. The idea of Arthur cheating on Merlin with Gwen seems rather untrue to the spirit of their relationships in the common Arthurian tales. But in the world of fanfiction, anything is possible.
Merlin might become even more protective of Arthur. Since Gwen is carrying Arthur's child, Merlin would see it as his duty to safeguard the future of Camelot. He could be more involved in Arthur's personal affairs, which might initially annoy Arthur but eventually he'd come to appreciate Merlin's concern.
It could be that Gwen unknowingly has some sort of magical influence over Arthur. For example, if there is some ancient magic tied to her lineage that she's not aware of, and it's causing Arthur to focus only on her and forget about Merlin. Another aspect could be that Arthur's love for Gwen is so overpowering that it's subconsciously pushing out other important relationships, like the one with Merlin.
Popular 'Merlin Arthur Gwen fanfiction' often has strong character development. It dives deep into the personalities of Merlin, Arthur, and Gwen, showing their growth and how they interact with each other. For example, it might show how Arthur becomes more accepting of magic because of Merlin and his relationship with Gwen.
In this type of fanfiction, it could start with Merlin having a prophetic dream where he sees himself kissing Arthur instead of Gwen. This dream haunts him and makes him realize his deeper, unacknowledged feelings for Arthur. As he tries to avoid this fate, circumstances keep pushing them together. Eventually, during a magical incident where emotions are heightened, Merlin accidentally kisses Arthur. This sets off a chain of events where they have to face the reactions of their friends and the court, and also explore their own feelings which they had previously buried deep inside.
One reason could be that in the fanfiction, Arthur and Merlin might have a deeper, more complex bond. Maybe they've shared more adventures and secrets that make Arthur value Merlin's presence more. Another possibility is that the writer might be exploring a different dynamic where Merlin offers something unique to Arthur that Gwen doesn't in that particular fictional scenario.
One great 'Merlin Arthur Gwen fanfiction' is 'The Bonds of Camelot'. It beautifully weaves the relationships between the three characters. It shows how Merlin's magic subtly influences the dynamic between Arthur and Gwen. Another one is 'A Triangular Affair' which delves deep into the complex feelings they all share for each other in a really engaging way.