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.
The main antagonist could be a new character created specifically for the fanfiction. This character could be a super - intelligent scientist from a highly advanced universe who has developed a technology that can nullify the powers of the Angels. He targets the Grand Priest because he believes that the current system of the multiverse, which the Grand Priest upholds, is unfair and restricts the progress of other beings. He wants to use his technology to overthrow the Grand Priest and create a new order in the multiverse. Another possibility is that a long - forgotten ancient race, with powers equal to or greater than the Angels, resurfaces and their leader decides to take on the Grand Priest to prove the superiority of their race.
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.
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 rogue Saiyan from a forgotten part of the Saiyan history. This Saiyan might have a grudge against the Z - fighters and their descendants, like Pan. He could have developed his own version of the Super Saiyan 4 power in a more evil way and wants to prove his superiority by defeating Pan.
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.
One idea could be exploring the Grand Priest's past. Maybe he had a different role in the universe long ago. Another could be a story where the Grand Priest has to make a difficult decision that goes against the will of the Omni - King. Or perhaps a story where the Grand Priest trains a new character to become a powerful being in the Dragon Ball universe.
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 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 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.
One interesting Grand Priest fanfiction could be one where he has a secret past that is slowly revealed. Maybe he was once a regular Saiyan who was chosen by the gods for a special purpose. This could lead to a whole new perspective on his character and his actions in the Dragon Ball universe.