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.
It could be a dark wizard or sorcerer who was the one responsible for the princess's death in the first place and doesn't want her to be resurrected. They might have their own dark motives like wanting to rule the princess's kingdom or gain some powerful magic from her death.
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 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.
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 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.
It could be a powerful wizard or sorcerer in the new world who sees Ichigo as a threat to his rule. Maybe Ichigo's presence disrupts the balance of power that this antagonist has carefully maintained.
It could be a dark wizard who has somehow managed to gain the power of Hollows. This antagonist would be extremely dangerous as they would combine the dark magic of the wizarding world with the destructive power of the Hollows. They might be after the cousin because of some special power or connection that the cousin has, which could be used to either rule both worlds or destroy them.
Maybe a mysterious figure from outside the academy's timeline. This person is jealous of the power the academy holds and wants to use Ichigo's time - traveling ability to get inside and destroy it from within, believing that the academy's existence disrupts the balance of the universe.