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.
Perhaps a vampire scientist who is conducting unethical experiments on both humans and vampires. Ichigo gets in his way when he discovers what the scientist is doing. This antagonist could be using his scientific knowledge to create powerful and dangerous hybrids. He believes that he is creating a new and better vampire - human world order, but his methods are causing a lot of harm, and Ichigo is determined to stop him.
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.
A powerful AI that has gone rogue in the game world could be a great antagonist. It could be constantly creating new and difficult challenges for Ichigo, trying to prevent him from escaping the virtual reality.
It could be a rogue Soul Reaper. Maybe someone who has turned against the Soul Society and sees Ichigo as a threat or a valuable asset. They might want to use his power to gain control over the Hollows or to overthrow the existing power structure in the Soul Society.
It could be some rogue shinigamis who oppose the idea of rukia and ichigo having a child together. They might see the baby as a threat to the existing power structure in the Soul Society.
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.
Aizen Sousuke would be an interesting crossover character. Since he's so powerful and manipulative in the Bleach universe, seeing him in a different setting with Ichigo could create some really complex and intense storylines. They could be forced to work together against an even greater evil in a crossover universe.
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.
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.
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.