In this 'naruto raised by snakes fanfiction', the main antagonists could be a group of rogue ninja who are jealous of Naruto's unique abilities. These rogue ninja might have had bad experiences with snakes in the past and see Naruto as an abomination. They would constantly try to undermine Naruto and his snake - raising mentors, leading to many battles and conflicts in the story.
There could be a powerful overlord in Hell who rules the place. Naruto's presence and his potential to grow strong might be seen as a threat to this overlord's power. So, this overlord would be one of the main antagonists, constantly sending minions or directly confronting Naruto to keep him in check.
The Hidden Leaf Village elders could be the main antagonists. They might see Naruto as a means to an end, a weapon to maintain the village's supremacy. So they push him to the limits without caring about his feelings or future.
One possible antagonist could be Orochimaru. He might see Erza as an interesting specimen to experiment on, due to her unique magic abilities. Another could be some of the dark guilds from Erza's world that cross over into Naruto's world and try to cause chaos.
One possible plot could be that Naruto is found by a powerful snake sage in the forest instead of the Hidden Leaf Village. The snake sage, seeing Naruto's potential, decides to raise him. Naruto then learns unique snake - related jutsus and a different set of ninja values compared to the ones in the Leaf. He might grow up with a more solitary and calculating nature due to the snake's influence.
Perhaps some traditionalist wizards in the Ministry of Magic. They might view Harry's training by snakes as a form of dark magic practice, even if it's not. So they become the antagonists, trying to arrest Harry or stop his training in some way.
The Titans could still be antagonists. Even though Percy is raised by Chaos, the Titans might see him as a threat to their plans of overthrowing the Olympians. They could try to capture or kill him to prevent him from interfering.
Maybe there are hunters in the fanfiction. Hunters who are dedicated to eradicating vampires and see Bella, as someone being raised by Dracula, as a prime target. They would pose a constant danger to her and Dracula.
Voldemort would still be a major antagonist. But there could also be new threats emerging from the clash of the wizarding world and Zabuza's world. Maybe some dark wizards who are envious of Harry's unique upbringing and new skills would try to take him down.
The Yonko would definitely be major antagonists. Since Luffy is in the New World, the likes of Kaido or Big Mom would be natural choices. Their vast power and influence would pose a great threat to Luffy and his crew.
It could be the same Voldemort as in the original series. But his approach to Harry might be different. Since Harry is being raised by Raven, he might be more difficult to target as he has different knowledge and skills. So Voldemort might have to come up with new and more devious plans to get to Harry.