Well, first of all, Luffy is a very emotional character. When he discovers Zoro's sacrifice, he would probably go into a state of shock initially. Then, his reaction could be explosive. He might not be able to accept it right away and would try to find a way to reverse the situation. He could also start blaming himself for not being there to protect Zoro, which would further fuel his determination to become stronger and protect the rest of his crew better in the future.
Obviously, Charlie and Miles are the main characters. Charlie is the one who has the big discovery about Miles being her father. And Miles is the person who is revealed to be Charlie's dad, so he must have a big role in the story as well, with his own backstory and reasons for why this situation has come about.
Perhaps Charlie overhears a conversation that makes her suspect Miles is her father. In the fanfic, the story could then focus on how Charlie approaches Miles to confirm this. There could be a lot of internal struggle for both of them. Miles might be hesitant to admit it at first, fearing how it will change their relationship. Charlie, on the other hand, might be desperate for answers and a real connection with her father she never knew she had.
The wizarding world would be in an uproar. Some would be sympathetic towards Harry, understanding the struggles he had endured. Others might be judgmental, especially those who come from pure - blood families and have different values.
It's possible that the baby's parents are from a rival pirate crew. In the 'luffy finds a baby fanfiction', this could lead to a lot of drama as Luffy has to decide whether to return the baby to its potentially dangerous family or keep it safe with his own crew. Or perhaps the baby's parents were victims of a powerful and evil force, and Luffy feels compelled to protect the baby from the same fate.
He might be shocked at first. In many fanfictions, Adrien would feel a mix of disbelief, anger, and sadness. He has always known his father as a strict but loving parent, and finding out he's Hawk Moth, a supervillain, would completely shatter his view of him.
In this fanfiction, perhaps Luffy might take on the role of a protector. He could be very confused at first, not knowing how to handle a baby. But then, his kind heart would lead him to try his best to keep the baby safe and find its family.
For Merlin, there are both good and bad consequences. On the bad side, some of the more traditional and anti - magic people in Camelot might try to oppose him. However, since Arthur now knows, he has a powerful ally. Arthur can protect Merlin from those who would do him harm. Also, Merlin can now be more open about using his magic for the good of Camelot, which is a positive change.