Well, in many fanfictions, Arthur might be overprotective of Merlin because of their deep friendship. They've been through a lot together, facing numerous dangers and challenges. Arthur has come to rely on Merlin in many ways, and he doesn't want to lose him. So, he becomes overprotective to ensure Merlin's safety.
One sign is that Arthur always keeps Merlin close by his side. He doesn't let Merlin take on overly dangerous tasks alone. For example, if there's a mission to a dangerous place, he'll either go with Merlin or send a lot of guards with him.
In fanfiction, Arthur might be rough with Merlin because of a prophecy or a pre - ordained fate. He might think that by being tough on Merlin, he can prevent some bad outcome that he believes is related to Merlin. For example, if there's a prophecy that Merlin will bring great change to Camelot, Arthur might be rough in an attempt to keep things as they are, out of fear of the unknown.
In Merlin fanfiction, Merlin calls Arthur sire as a sign of respect. In the Arthurian legend and related fictional settings, sire is a term used to address a nobleman or a person of higher status. Merlin, being Arthur's servant and advisor in many versions, uses this term to show his loyalty and deference to Arthur's position as king or prince.
One reason could be related to Arthur's relationships. If Arthur has a love interest or close friendships that Merlin wishes he could have with Arthur too. Like, if Arthur is spending a lot of time with a particular noble or a lady, Merlin might be jealous because he wants that same level of closeness with Arthur.
One reason could be for the continuation of his dynasty. In the context of Camelot, having a son would mean there is an heir to the throne. It would ensure the stability and future of his kingdom. Without a son, there could be disputes and power struggles over who will succeed him.
The banter between Merlin and Arthur in fanfiction is really interesting. It can range from light - hearted jabs at each other's habits. Merlin might poke fun at Arthur's love for jousting and how he always gets a bit too competitive. Arthur could retort by mentioning Merlin's clumsy attempts at serving him. Their banter also reflects the underlying respect they have for each other despite their different social standings. It's like a friendly sparring that keeps their relationship dynamic.
It shouldn't be allowed because it's a form of abuse of the characters. Arthur and Merlin are characters with their own identities and storylines, and sexual assault fanfiction distorts them in a very wrong way.
It's appealing because it takes two beloved characters and gives their relationship a new dimension. Fans love to see the different ways their dynamic can be explored, especially in a romantic context.
The 'Redemption' trope is also seen in some fanfictions. Maybe Arthur has made some mistakes in the past, and Merlin helps him to redeem himself. Or vice versa. It's a way to explore the characters' growth and development. And then there's the 'Battle Against Evil' trope where they fight side by side against dark forces, which really showcases their bond and loyalty.
I really like 'The Sword in the Stone' as a fanfiction - like take on Merlin and Arthur. It shows Merlin as this wise and mysterious figure guiding Arthur on his journey to become king. There are also some great interactions between the two characters that show the deep bond they share. You can find it in many libraries or online bookstores.