Perhaps it's a corrupted version of one of the Kais. They see Gohan's meeting with Zeno as a threat to their own power and status in the universe. So they try to sabotage their relationship and cause chaos between them.
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.
There might be a new powerful entity that emerges from the void between universes. This entity is jealous of Zeno's power and sees Gohan as a way to get to Zeno. So it tries to manipulate Gohan into doing its bidding against Zeno.
Perhaps Gohan meets Zeno during a multiverse tournament. Gohan, being the smart and powerful warrior he is, could start a conversation with Zeno about the importance of protecting all universes, not just for the sake of power but for the life forms in them. Zeno might be intrigued and start to see the universes in a new light.
It could be a new villain from the future that somehow follows future Gohan back in time. This villain might be an evolved form of an existing enemy, stronger and more cunning. They could be after the secrets that future Gohan holds about the future technology or the power levels of the Z - fighters in the future.
It could be a new villain created specifically for the fanfiction. Maybe a powerful sorcerer who wants to steal the Saiyan's power.
A powerful dark wizard or sorcerer could be the antagonist. He might be after the Saiyan's life - force or some special energy that Gohan and Bardock possess. This wizard could use dark magic to create all sorts of obstacles for them, like illusions, cursed landscapes, and powerful minions.
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.
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.
The androids could still be the main antagonists. Even if they have been defeated in some timelines, in a fanfiction, they could be brought back stronger or with new abilities. Their cold and ruthless nature would make them a great challenge for Gohan and Trunks to overcome.
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.
Maybe a powerful demon who has the ability to possess animals. He uses this power to send hordes of possessed animals against Gohan. His goal is to drain Gohan's energy and use it to break free from a magical seal that has been confining him for centuries. He sees Gohan as a threat to his plan because of Gohan's strength and connection to the Dragon Balls.
It could be that a new threat emerges. Maybe another android, stronger than Cell, was observing the fight and decides to take advantage of the chaos after Gohan's death. This new android could become the main antagonist as it poses a greater danger to the remaining fighters.