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.
A corrupt zookeeper could be the main antagonist. He discovers a way to control animals using dark magic. He decides to target Gohan because he fears Gohan will expose his evil deeds. He uses the animals in his zoo, which he has turned into fierce fighters, to attack Gohan and keep him away from the truth.
It could be a new Saiyan from another part of the universe who is jealous of the power the family holds. He might come to Earth to challenge the son and prove his own superiority.
Maybe an evil Namekian. Namekians have unique powers, and an evil one could use his abilities to manipulate the life energy in a way that creates zombies. He might be jealous of the Saiyans' power and wants to use the zombies to wipe them out. He could also be in cahoots with some of the other villains from the Dragon Ball series, like Cell. The heroes would have to not only fight the zombies but also figure out this Namekian's motives and how to stop him.
Super 17 himself could be the main antagonist if the fanfiction focuses on his corruption. He could be consumed by power and decide to rule over the universe. His already - formidable powers would make him a terrifying opponent for the Z - fighters, and they would have to find new ways to stop him without relying too much on their usual techniques.
Maybe an evil scientist who has created a new form of bio - weaponry. He targets Gohan and Lime because they pose a threat to his plans of world domination. His creations could have abilities that are a challenge for Gohan and Lime to counter.
In a crossover with the Avengers, Iron Man could be an ally. His high - tech gadgets combined with Gohan's power would be a great team. Also in the Pokémon world, Ash could be an ally. Ash's knowledge of Pokémon and Gohan's combat skills could be useful. In the Bleach universe, Ichigo could be a strong ally for Gohan, with their combined sword - fighting and energy - blasting abilities.
A powerful wizard or sorcerer who wants to use Goku's daughter's pure Saiyan energy for some dark magic. He could be creating chaos on Earth, and she has to stop him before he drains all her energy. This antagonist would be a departure from the typical physical - strength - based villains in Dragon Ball, adding a new dimension to the fanfiction.
It could be Orochimaru. In this fanfiction, Orochimaru might hear rumors about the Dragon Balls and their power to grant wishes. Since he is always seeking power and immortality, he decides to hunt for the Dragon Balls. However, because Naruto is so focused on his own battles against Orochimaru in the ninja world, he fails to realize that Orochimaru's new goal involves the Dragon Balls and the entire Dragon Ball Z universe could be at risk.
It could be another powerful being from outside the known Dragon Ball universe. Maybe a being from a parallel dimension who sees the Grand Priest as a threat to his own plans for universal domination. Another option could be a rogue Angel who has turned against the Grand Priest and the current order of things in the multiverse.
It could be a new character created specifically for the fanfiction. Maybe a powerful alien warlord who has been observing the Saiyans and Broly from afar. He decides to test his strength against them by luring them into a trap. This antagonist could have unique powers, like the ability to manipulate time on a small scale during battles.
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.