In this fanfiction, the Church or religious authorities in Camelot could be the main antagonists. Vampires are often associated with the devil or evil in religious beliefs. So, they might see Merlin's transformation into a vampire as a sign of him being corrupted by dark forces. They would try to exorcise or destroy him, not understanding that he's still the same Merlin at heart, just with a different form and new powers.
There could be a new dark sorcerer who emerges. This person might be jealous of Merlin's position as king and a powerful sorcerer. They would try to bring chaos to the kingdom and usurp Merlin. Also, the neighboring kingdoms that are envious of Merlin's prosperous kingdom could send spies or even start wars, acting as antagonists.
Morgana could be a main antagonist. She has her own desires for power and may see Merlin's rise to kingship as a threat to her plans. So she tries to stop him at every turn, using her dark magic against him.
The main antagonists could be the traditional enemies of Camelot like Morgana. She might see Merlin's act of adopting Mordred as a threat to her plans. Also, some of the knights or nobles in Camelot who are superstitious and believe Mordred is a danger just because of the prophecies could be antagonists. They might try to separate Merlin and Mordred or even harm Mordred.
In some cases, there could be a rogue vampire with a vendetta against the Cullen family. This vampire might target Alice and Jasper specifically because of their unique abilities and importance within the family. They could try to disrupt their relationship or harm them in other ways.
Often, other vampires can be the main antagonists. Maybe there are some elder vampires who don't like Yugi and Atem's new - found power or their different way of being vampires. They could see them as a threat to the traditional vampire ways.
The Soul Reapers could be antagonists if they view Ichigo's vampirism as a threat to the balance between the living and the dead. They might try to hunt him down to eliminate this new, unregulated force.
Other vampires might be his main antagonists. They could see Issei as a threat, perhaps because he's a new vampire with different ideas or powers. Or they might want to use him for their own gain.
There could also be a powerful vampire hunter. This person might have a personal vendetta against vampires in general or specifically against Ichigo. They could use all kinds of weapons and tactics to try and kill him.
Often, other vampire clans that oppose Bella's transformation can be the main antagonists. They might see her as a threat to the vampire world order. For example, a traditionalist clan that believes humans should not be turned so easily could try to harm Bella or drive her away from Edward.
A group of vampire hunters could also be the antagonists. They might not be able to tell the difference between Issei and a full - blooded vampire and target him. They could be very zealous in their mission to wipe out all vampires and view Issei as just another one to be hunted down.