Cell could be a great antagonist. Even if Gohan goes back in time, Cell might still find a way to exist. Maybe he adapts his plan to absorb the Androids earlier in time. Gohan would then have to face a more powerful and unpredictable Cell, who could potentially change the timeline to his own advantage.
An interesting choice for the main antagonist could be an alternate version of Gohan himself. If time travel causes some sort of split in the timeline, an evil Gohan could be created. This evil Gohan might have different goals, like ruling the universe, and would oppose the 'good' Gohan at every turn, leading to some intense battles and moral dilemmas for our hero.
It could be a rogue Saiyan from another part of the universe who somehow manages to enter the hyperbolic time chamber. This Saiyan might have a grudge against Gohan's family or be on a quest for power and sees Gohan as an obstacle.
Frieza would be a great antagonist. If Goku goes back in time to a point before Frieza was defeated, Frieza could be an even more menacing force as Goku might not have all the power - ups he had in the original timeline. He'd have to rely on his wits and basic training more.
Cell could also be a possible antagonist. Even though he was defeated, if the Saiyans are brought back, it could disrupt the balance that was achieved after his destruction. He might be revived or some remnant of him could emerge to oppose the Saiyans' return.
It could be a new villain who is specifically targeting Gohan because of his new form. This villain might see Gohan's transformation as a sign of weakness or an opportunity to mess with the Z - fighters. Maybe they want to use Gohan's confusion and the chaos it causes among his friends and family to their advantage.
It could be someone who doesn't want Gohan and Zeno to form an alliance. Maybe a rogue god from another universe who fears that if Gohan influences Zeno, his own plans for universal domination will be thwarted.
It could be Madara. Even if Obito goes back in time, Madara's long - term plans for the ninja world are still in motion. He might be the main force that Obito has to oppose as Madara's ideas often lead to destruction and chaos.
It could be an evil time - traveling sorcerer. He wants to rewrite history in the prehistoric era so that he can rule the world in the future. He sees the Dino Thunder Rangers as a threat to his plans and tries to stop them from interfering.
It could be a time - traveling version of Orochimaru. He might have his own plans for the past and Sasuke's interference could put them at risk. So, he tries to stop Sasuke from changing the past in a way that would undermine his future goals.
It could be a time - traveling version of Zeref. He might be trying to stop Natsu from changing things in the past that could potentially lead to his own downfall in the future.
It could be an ancient and powerful Titan who was thought to be defeated long ago. Since Percy is in the past, this Titan could be in his prime and see Percy as a threat to his plans of ruling over the world again. For example, Kronos might be even more powerful in this past era and Percy has to find a way to stop him without the help of his modern - day friends.