Another possible antagonist could be an ancient vampire who sees Ichigo as a threat to his own power. This old vampire might try to either turn Ichigo to his side or destroy him to maintain his dominance in the vampire world.
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.
There could also be a group of humans who know about the existence of vampires and form a sort of vampire - hunting organization. They don't care that Ichigo might be different from other vampires and just see him as a monster to be eradicated.
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 some fanfictions, a powerful Hollow could be the main antagonist. It might target Orihime because of her special powers and Ichigo has to protect her while also dealing with his new - found feelings for her. The Hollow could be constantly attacking them, trying to separate them.
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.
Often, Aizen can still be a major antagonist even in time travel fanfictions. His schemes might extend across time, and Ichigo and Orihime have to figure out how to stop his plans that are messing with different timelines. Another could be the more powerful versions of Hollows from different time periods. They might be stronger and more menacing than the ones Ichigo and Orihime are used to in their original timeline.
Some fanfictions might introduce new characters as antagonists. These could be rogue Quincy who believe that Ichigo and Orihime's relationship is a threat to their plans for the spiritual world. They try to separate them to gain an upper hand in their schemes.
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.
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.
The main antagonists could be other powerful Hollows who don't like Grimmjow having a human - like Ichigo as a pet. They might see it as a sign of weakness or an abnormal behavior and try to attack Grimmjow to get rid of Ichigo.
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.