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.
Well, in the Issei is a vampire fanfiction, his main antagonists could be a group of ancient vampires who have a strict code that Issei is breaking. They might see his existence as a disruption to their carefully maintained vampire society. Additionally, there could be a human organization that has discovered the existence of vampires and is determined to wipe them out. They could be using advanced technology and strategies to hunt down vampires, and Issei would be right in their crosshairs. There might also be a vampire who turned Issei against his will and now wants to control him, which would make that vampire a major antagonist in Issei's story.
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.
It could be an old enemy of the group who has a personal vendetta against Issei. This antagonist might be jealous of Issei's power or popularity within the group and plots to turn everyone against him. They could spread false rumors or create false evidence to make it seem like Issei has betrayed the group.
Perhaps it could be a group of bullies from Issei's school days. They used to pick on him and now he's coming back to make them pay.
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.
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.
Another vampire, an older and more powerful one, could be an antagonist. This vampire might see Percy as a threat to his power or territory. Maybe he wants to either kill Percy or make him submit to his rule.
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.
In certain fanfictions, fallen angels may act as antagonists. They could be trying to disrupt the relationship between Akeno and Issei or simply cause chaos in the devil world where they live.
The vampire Authority could be antagonists. They might see Sookie as an abomination since she was a fae - human before being turned into a vampire. They could try to eliminate her to maintain the 'purity' of the vampire race according to their own rules.
One possibility is a group of shinobi from a rival village. They believe that Naruto's mother held the key to some great power or secret that could give their village an edge. So, they try to either capture Naruto to extract the information or stop him from uncovering more about his mother. Another could be a dark spirit that was somehow sealed away by Naruto's mother long ago, and now it's trying to break free and take revenge on her through Naruto.