Perhaps a new character created specifically for this fanfiction. This new antagonist could be someone who wants to use the power of reincarnation and time travel for their own evil purposes, like trying to rewrite the world to their liking and is opposed by Ichigo and Kaien.
There's a possibility that an old enemy from the past, who was thought to be defeated, returns due to the time travel. This enemy could have new powers or a different agenda this time. They might see Ichigo and Kaien's reincarnation as a threat to their plans and thus become the main antagonist. Also, it could be that the very act of time travel and reincarnation creates an imbalance in the spiritual world that gives birth to a powerful and malevolent force, which then takes on the role of the main antagonist.
One interesting concept could be how Ichigo's character might change in the reincarnation and time travel scenario. Maybe he has memories of his past life that influence his actions in the new timeline.
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.
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 could still be the main antagonist. Even if Ichigo travels back in time, Aizen's long - term plans might be already in motion. Ichigo would have to find a way to expose and stop Aizen earlier than in the original timeline.
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.