He might also get angry. Merlin has his own pride and after all he has done for Arthur, being insulted like this would make him furious. He could start yelling back at Arthur, listing all the times he has saved Arthur's life or the kingdom.
She might be completely overjoyed. With tears of happiness in her eyes, she'd immediately say yes and throw her arms around Merlin. After all, she loves him deeply and has probably been waiting for this moment.
She might be shocked at first. Given that in the original story Morgana has her own complex relationship with magic, seeing Merlin's magic could stir a mix of emotions in her, like envy or a sense of relief that she's not alone in her magical abilities.
In some versions of this type of fanfiction, Arthur's reaction is complex. He might be stunned into silence when he first finds out. His mind would be racing with thoughts about all the times Merlin has used his magic to save him or Camelot without his knowledge. After the shock subsides, he may have a heart - to - heart with Merlin. They would talk about the implications of the secret, and Arthur would likely start to view Merlin in a whole new light, as not just his servant but as a powerful ally with a great burden.
One of the top fandoms for fanfiction is the Doctor Who fandom. With its time - traveling and space - exploring premise, the possibilities are endless. You can create new adventures for the Doctor and his companions, or introduce new alien species. The Supernatural fandom is also very popular. There are so many different mythologies and creatures in the show that you can base your fanfiction on. And then there's the Percy Jackson fandom. It combines Greek mythology with modern - day settings. You can write about new quests for demigods, or explore the relationships between different characters from different cabins at Camp Half - Blood.
He might be surprised. The Harry Potter world has its own set of rules and characters that are very different from what Merlin is used to in his Arthurian world.
The more superstitious knights might think that Merlin has called upon some real evil spirits and become very worried. They might start praying or asking Merlin to make the 'spirits' go away. And some might be so scared that they avoid Merlin for a while afterwards, not wanting to be the target of his next scare.
Merlin would probably be very conflicted. On one hand, he is loyal to Arthur and Camelot, but on the other, being forced into a marriage is a huge infringement on his freedom. He might be scared about how this will change his life and his ability to use his magic secretly. Arthur, on the other hand, could be initially resistant as well. He sees Merlin as a friend and a servant, and this sudden change to a marital relationship would be strange for him.
He could be very surprised at first. Maybe he's just out on a normal day in the forest and suddenly comes across a baby. His immediate reaction might be to look around for any sign of the baby's parents.