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.
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.
There might be some great 'arthur forgets merlin' fanfictions on Wattpad as well. You can look for ones with high ratings and lots of reviews. Usually, those are the ones that are well - written and engaging. It could be a story where Merlin has to prove himself all over again to Arthur, which would be quite an interesting plot.
One interesting 'arthur forgets merlin fanfiction' could be where Merlin has to use his magic in new and secretive ways to re - acquaint Arthur with their friendship. Maybe he creates small magical incidents that jog Arthur's memory in a subconscious way.
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.
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.
It would become much more complex. They might start off being confused and a bit awkward around each other. But then, as they start to explore their feelings, they could become closer in a more intimate way. Their friendship would be transformed into something deeper, with a new level of understanding and trust.