The other powerful villains in the Dragon Ball universe could be the main antagonists. For example, Cell or Majin Buu. These characters are extremely powerful and would likely oppose Frieza (Harry). Cell, with his ability to absorb other beings to gain power, would be a formidable foe. And Majin Buu's unpredictable and destructive nature would also put Harry, in his Frieza form, in a lot of danger.
It could be the Saiyans. In the Dragon Ball universe where Frieza exists, the Saiyans are often his enemies. Even if Harry is reborn as Frieza, the Saiyans' natural strength and warrior spirit would pose a great threat to him. They might not understand his new - found combination of magic and Frieza's powers and see him as an even greater danger.
There could also be a trainer who is jealous of Harry's unique abilities, being reborn from the Harry Potter world. This trainer might try to undermine Harry at every turn, stealing his Pokemon or sabotaging his battles, much like Draco Malfoy was often in conflict with Harry in the original Harry Potter series.
The main antagonists could still be Voldemort and his Death Eaters. Just because Harry is being raised by Moriarty doesn't mean Voldemort's threat disappears. In fact, Moriarty might see Voldemort as either a rival or a potential ally, which would add another layer of complexity to the story.
It could be the Death Eaters still. But with Harry being raised by Revy, they might face a much more formidable opponent. Harry would have a different set of skills and a more aggressive approach, so the Death Eaters would have to adapt. Another possibility could be some of the more corrupt wizards in the Ministry of Magic who don't like this new, unpredictable Harry.
Voldemort would still be a major antagonist. His goal of ruling the wizarding world and eliminating Harry Potter would remain. However, there could be new antagonists added due to Hawkgirl's presence. For example, some Thanagarian villains might follow Hawkgirl to the wizarding world and see the chaos Voldemort is causing as an opportunity to gain power for themselves. They could try to form an alliance with Voldemort or simply try to take over the wizarding world on their own.
Well, perhaps Harry, reborn as Frieza, would use his wizarding knowledge in a different way. He could combine magic with Frieza's powers. Maybe he'd use spells to enhance his energy blasts or use his knowledge of potions to create new substances that could boost his strength even more in his new Frieza form.
Definitely Voldemort and Sauron. Voldemort would see the One Ring as a powerful tool to conquer the wizarding world completely. Sauron, on the other hand, would not want anyone else to have control over the ring. So they would both be after Harry and the ring, making them the main antagonists.
In this fanfiction, the Death Eaters would remain as antagonists on the wizarding side. They follow Voldemort's ideology and would oppose Harry no matter who raised him. But also, there could be elements from Jean Grey's world that turn against Harry. For example, if there are mutants who fear that Harry's magic could disrupt the balance between mutants and humans, they might become antagonists. It would add a whole new layer of conflict to the story.
Sauron would surely be one of the main antagonists. His lust for power and domination over Middle Earth is well - known. In this fanfiction, he would see Harry Potter as a major threat to his plans.
In this kind of fanfiction, the main antagonists could be a combination of the traditional Harry Potter villains like the Death Eaters and new threats from the Marvel side. For instance, Dormammu could somehow sense Harry's potential and try to either possess him or destroy him to prevent him from becoming a powerful force against him. And of course, there would always be some wizards in the Harry Potter world who are against any kind of change or outside influence, and they could also oppose Harry.
Another possible antagonist could be some dark wizards who are trying to use Harry's new identity for their own evil purposes. They might try to manipulate him or use his connection to the Blacks to gain access to powerful magic or influence within the wizarding world.