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 possible plot could be that Arthur and Gwen face challenges in raising their baby in a world filled with political intrigue. Maybe there are those who see the baby as a threat or a tool for power. For example, some nobles might try to use the baby's influence to gain more land or favors. Arthur and Gwen would then have to protect their child while also ruling Camelot fairly.
The antagonists could be forces of dark magic. They might sense the power within the baby and want to either corrupt it or use it for their own evil purposes. For example, there could be a sorcerer who wants to steal the baby and raise it to be his minion. Also, some neighboring kingdoms that see Camelot's new heir as a future threat might send spies or assassins to target the baby.
The baby could be a key to uniting two warring factions in the Arthurian world. Arthur and Gwen would then have to navigate the political and social implications of this. They could have to protect the baby from those who would use it for their own gain. Or perhaps the baby has a unique connection to the magical elements of the land, like being able to make plants grow instantly or control the weather. This would attract the attention of both good and bad forces, and Arthur and Gwen would have to be vigilant.
They can develop their relationship through shared experiences. For instance, taking the baby on a picnic in the forest and teaching it about nature. Arthur could show the baby how to fish, while Gwen tells it stories about the stars. Another way is through protecting the baby from threats. When danger looms, their parental instincts kick in, and they become closer as a family. Also, they can develop their relationship by celebrating the baby's milestones, like its first steps or first words, which would create precious memories for the family.
His magic would be crucial in safeguarding the baby. If there were any threats from dark magic, Merlin could sense them early on. He could then use his powers to counteract the dark magic, perhaps by creating a magical shield around the baby. Also, if the baby were to be born with some sort of magical ability (since Arthur and Gwen are in a magical world), Merlin could help the baby control and understand its powers as it grows up.
One of the themes is leadership. Since Arthur is a king, many fanfictions explore how Gwen influences his leadership style and vice versa. They are often depicted as a power couple, making decisions together for the betterment of their people. Also, the theme of sacrifice is present in some fanfictions. Either Arthur or Gwen might sacrifice something for the other's well - being or for the good of the kingdom.
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.