Another possibility is a character who has been possessed by an evil spirit. They may not be in their right mind when they stab Merlin. This could add an element of mystery as Merlin has to figure out not only who stabbed him but also how to exorcise the spirit from the person's body.
It could be a jealous rival. Maybe someone who is envious of Rito's status or abilities and decides to take him out by stabbing.
It could be an old enemy of Nicole's from her police work. Maybe someone she put in jail before who has a vendetta against her. This person might have been released recently and is out for revenge.
It could be an enemy ninja from another village who wants to take out Konoha's strongest shinobi. Maybe someone from the Sound Village still holding a grudge against Naruto.
Morgana could be a likely antagonist. She has been at odds with Merlin in the series and has her own motives and powers. She might see stabbing Merlin as a way to further her plans against Camelot or to eliminate a threat to her own power.
Arthur could be the one to spank Merlin. Given their relationship where Arthur is the king and Merlin is his servant, if Merlin does something that Arthur believes is a serious offense against Camelot or his own authority, he might take this rather unorthodox form of punishment. Arthur is known for his hot - headedness and quick decision - making, so in a fit of anger, he could do it.
It could be a jealous gamer. Someone who has been envious of the Game Shakers' success and sees Hudson as an easy target to get back at them.
Another character who has a personal vendetta against Clary or her family. For example, a jealous rival or someone who blames her family for past misfortunes. This would add a layer of drama and personal conflict to the fanfiction.
It could be an old enemy seeking revenge. Maybe a powerful Titan who escaped Tartarus and blames Percy for his previous defeat. This Titan could have been plotting in secret, waiting for the perfect moment to strike and finally end Percy's life.
Another likely attacker could be a vengeful character from Naruto's past. Maybe someone whose family or village was affected by Naruto's actions, even if those actions were for the greater good. This person could be seeking revenge and thus beat Naruto up in the fanfiction.
Maybe it's a new villain who is trying to make a name for themselves. This villain could see shooting Captain America as a way to gain instant notoriety in the criminal underworld. They might have studied Captain America's weaknesses and found an opportune moment to strike.