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.
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.
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.
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.
There could be a group of corrupted Quincy who view Ichigo's resurrection as an abomination. They believe that his power goes against the natural order of things. So, they band together to try and eliminate him. They might have some new and dangerous weapons or techniques specifically designed to counter Ichigo's resurrected powers. This group could be led by a very powerful Quincy who has a personal vendetta against Ichigo or the Soul Reapers in general.
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.
A powerful sorcerer could be the main antagonist. This sorcerer might be after the same power source that Ichigo and Jackie are either protecting or trying to use for good. The sorcerer could be using dark magic to create chaos in their world and trying to stop Ichigo and Jackie from achieving their goals.
There could be a group of anti - hybrid fanatics. They believe that the existence of a Naruto - Ichigo descendant is an abomination that goes against the natural order of both worlds. They form a secret society and start causing chaos, trying to eliminate the descendant. Their leader could be a former ninja who lost his family in a war and blames the mixing of powers for all the world's problems. They would be a complex antagonist as they have their own warped ideology to drive their actions.
Maybe a giant lemon - themed monster. This monster is created by some dark force that wants to disrupt the balance between the two worlds in the crossover. It has the ability to spit out acidic lemon juice that can corrode anything. Ichigo has to figure out its weakness and defeat it to restore peace. The monster could be controlled by an unseen force that is the real antagonist behind the scenes.
A powerful Hollow that has the ability to disrupt Ichigo's gamer powers could be the main antagonist. This Hollow might be able to hack into Ichigo's power system, so to speak, and cause glitches in his abilities, making it difficult for him to fight.
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.