Maybe it could be the Duke of Nuts. He might be jealous of Breezy's popularity or her special abilities. So he tries to frame her for something she didn't do in the Candy Kingdom, causing chaos for her and making her have to prove her innocence.
The Ice King could be a great antagonist. He might be jealous of the Water Princess's power over water and try to freeze all the water in her domain. His craziness and his desire for power could drive him to do this.
One idea could be that Breezy gets lost in the Land of Ooo and Finn and Jake have to help her find her way back home. Maybe she has a special mission that she needs to complete but can't without their help.
It could be a remnant of the Ice King's past enemies. Maybe one of the magical creatures that the Ice King used to annoy or fight against now sees Simon as a threat. Since Simon has the knowledge and power of the Ice King, this creature wants to stop him before he becomes too powerful again.
The Lich could be a great antagonist. He might frame Finn as part of his plan to spread chaos and death in Ooo. Since the Lich is always looking for ways to disrupt the peace, using Finn as a scapegoat would be a clever move for him.
It could also be a corrupted version of one of Fionna's friends. For example, if Cake were to be turned evil by some dark magic. Cake would know all of Fionna's weaknesses and could use that knowledge against Finn in his Fionna form, making her a formidable and tragic antagonist.
It could be a rogue ninja who has mastered a forbidden jutsu during the time skip. This ninja might have been a former ally or a student from one of the ninja academies who turned to the dark side. His goal could be to take over all the ninja villages using the power of the forbidden jutsu.
It could be Orochimaru. Since Orochimaru has always been interested in power and immortality, if Naruto goes back in time, Orochimaru might sense the change in the timeline and try to stop Naruto from changing things that could affect his future plans for power.
A time - traveling pirate who has his own agenda and wants to change the course of pirate history for his own gain could be the antagonist. This pirate might be jealous of Luffy's future achievements and try to stop him from ever becoming the Pirate King by changing things in the past.
It could be a future version of an existing villain who has also found a way to time - travel. For example, a more powerful and cunning Orochimaru from a future timeline where he has mastered even more dark jutsu. He might try to stop Naruto from changing the past in ways that would prevent his own rise to power.
Even some of the ancient demigods who turned bad could be antagonists. They might see Percy as a threat to their power or plans in the past. They could have different values and goals compared to Percy, leading to conflicts. For example, a demigod who was power - hungry and wanted to use time - travel to change things for his own gain, and Percy would have to stop him.