Love and loyalty are always big themes. Their deep love for each other is explored, how they stay loyal through thick and thin. In some fanfictions, there's a theme of sacrifice, where one of them might be willing to sacrifice something important for the other's well - being. And cultural differences between them can also be a theme, as they come from different backgrounds, and how they bridge those gaps in their relationship.
Friendship is a very common theme. Their bond is often explored, like how they trust each other unconditionally. For example, in many fanfictions, Gwaine will always have Merlin's back in dangerous situations.
Some fanfics may focus on the concept of female empowerment. The female Merlin could be portrayed as a stronger or more assertive character compared to her male counterpart. Gwaine might be inspired by her strength and they could team up to fight against injustice or magical threats in Camelot. This gives a new spin on the traditional Arthurian legends.
Another theme is loyalty. They are loyal to Camelot and to each other. In many stories, this loyalty is tested, for example, when they have to choose between following the rules of Camelot and doing what they think is right. It makes for some really interesting plotlines. And often, there's also the theme of adventure. They might go on quests, fight monsters, or discover new magical things, which adds a lot of excitement to the stories.
One common theme is friendship. Since Gwaine and Merlin are already friends in the original series, the OC often comes in to either strengthen their friendship or create some sort of conflict within it. For example, an OC might be jealous of their bond at first.
One common theme is loyalty. Gwaine is always shown to be loyal to Merlin, no matter what. Another theme is adventure. They often go on adventures together, which strengthens their bond. Also, trust is a big theme. Merlin trusts Gwaine with some of his secrets, and Gwaine respects that trust.
Friendship is a very common theme. They often start as acquaintances and grow into very close friends. Another theme is adventure, as they usually go on quests or face some sort of danger together.
One great fanfiction could be where Gwaine, as king, has to navigate the political waters while being married to Merlin. Their relationship might bring about new laws that are more inclusive and magical - friendly in the kingdom. Maybe Merlin uses his magic secretly to help Gwaine make the best decisions for their people.
The plot might also be about a threat to the kingdom. As king, Gwaine has to lead the defense, and with Merlin as his partner, they use magic and strategic thinking. Their marriage is not just a love story but also a partnership in ruling. They might face an ancient evil that only Merlin's magic can defeat. Along the way, they'd grow closer, and their relationship would be tested and strengthened as they save the kingdom.
Well, there could be themes of responsibility. With Merlin being pregnant, Gwaine would have to step up and take on new responsibilities. There might also be a theme of facing society's reactions. If this is set in the Merlin universe, the medieval - like society might have strong opinions about an unmarried pregnancy. Additionally, there could be a theme of friendship being tested, as their friends might react in different ways to this situation.
One great gwaine and merlin fanfiction is 'The Adventures of Gwaine and Merlin'. It has a lot of action and their friendship is beautifully portrayed. They go on quests together and face various challenges, which really shows their bond.