There could be an evil scientist who has been observing Broyl and Goku. He creates powerful bio - mechanical creatures to fight them. His goal could be to capture Broyl and Goku to study their powers and use them for his own evil experiments and world - domination plans.
It could be a powerful demon from another dimension. This demon could sense the power of Broyl and Goku and see them as a threat to his plans of universal domination. So, he decides to target Earth, where they are, to try and eliminate them.
Well, it could be about Broyl and Goku having to face various threats on Earth together. Maybe they start off as rivals but then realize they need to team up to defeat powerful villains. For example, they might encounter an alien race that wants to take over Earth. Their combined strength and different fighting styles could make for an exciting story.
The main antagonist could be a rogue ninja who has infiltrated the village's council. This ninja could be secretly manipulating things behind the scenes to get Naruto sent to prison. He might have a vendetta against Naruto for some past encounter or just see him as a threat to his own evil plans.
Perhaps one of the Titans who were banished to Tartarus. They might see Percy as a threat to their plans of revenge or escape. Kronos, if he still has some form of existence in Tartarus, could be a major antagonist. He would have his own motives and powers, and he could try to manipulate Percy or simply get rid of him.
It could be an evil scientist who created the Chichi clones for some malicious purpose. Maybe they want to use the clones to control Goku or to disrupt his life so he can't protect the Earth.
It could be Zamasu. Since Goku Black was Zamasu in Goku's body in the original story, perhaps a more powerful and enraged Zamasu could be the main antagonist. He might be jealous of the new fused form's power and seek to destroy it.
Perhaps it's a dark spirit that has been trapped in the Acrobleno for centuries. When Harry arrives, it senses his power and wants to possess him or use his magic for its own evil purposes. So it becomes the main antagonist, constantly trying to harm Harry and stop him from leaving.
There could also be a powerful magical creature that has been corrupted by dark magic. This creature might be feared by all in the past, and when Harry arrives, it sees him as a new target or a potential source of power. Harry has to figure out how to defeat it using his own skills and the limited resources available in the past.
It could be the Devil or some powerful demon lord in hell. This antagonist would try to keep Naruto trapped in hell and prevent him from escaping. He might see Naruto as a threat to his rule in hell or just enjoy tormenting him.
One possible main antagonist could be the ruler of hell. This powerful being would see Naruto as an intruder and a threat to the order of hell. Another could be a demon who has a vendetta against Naruto for some reason, perhaps because Naruto's positive energy is anathema to the demon's evil nature.
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.