The father's reaction varies. In a particular fanfiction I read, the father was first in disbelief. He couldn't believe that the young man in front of him was his son Merlin. Then, as Merlin showed some signs or told some stories that only his son would know, the father became overjoyed. He started to tell Merlin about their family history that Merlin didn't know before, which was a very emotional and bonding moment between them.
One possibility is that Merlin's revelation of his magic could lead to a change in his relationships. For example, his friends who were previously unaware might react with shock, some with fear and some with acceptance. His enemies, on the other hand, would likely see him as an even greater threat and might plot more vigorously against him.
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.
Uther might initially be shocked. He has always been so against magic, so he could be furious at first. He might feel betrayed by Merlin, whom he has known for a while.
The storyline might progress with a lot of drama. People's reactions would be the key factor. If he reveals it to Arthur, for instance, Arthur's response could either make their friendship stronger or break it apart. This would then lead to different paths for Merlin, either having to hide again or standing up against all odds.
Meeting his father could have made Merlin more confident. Knowing his family background and having his father's presence (even if it was just in the fanfiction) might have given him a sense of belonging. He could also become more powerful, as perhaps his father passed on some knowledge or power to him.
Well, Arthur could feel betrayed at first. He has been brought up to believe that magic is bad, and here his closest friend has been hiding this big secret. However, as he realizes how Merlin has saved his life and the kingdom many times with his magic, he might come to accept it and even see the potential of magic being used for the good of Camelot. For example, Merlin's magic has defeated many enemies secretly, which Arthur might now understand as a necessary protection for the kingdom.
Yes, in some fanfictions, Merlin is written as Mordred's father. Writers often like to explore different family relationships and backstories in fanfiction, and this is one such interesting take on their relationship.
Maybe it's a mysterious figure from Merlin's past. Perhaps he had an encounter with a powerful magical being, and some kind of spell or pact led to the pregnancy. It could be a long - forgotten event that now resurfaces with this unexpected pregnancy.
He'd be overjoyed. Naruto always wanted to know his parents. Meeting his father earlier would give him a sense of belonging and pride. He'd probably pester his father with a million questions about his mother, his family, and their abilities.