It could be that Akatsuki members are the main antagonists. They might view Erza as a threat to their plans, especially if she starts to gain power under Naruto's tutelage. Also, there could be some jealous ninja from Naruto's own village who don't like the idea of an outsider like Erza getting special training from Naruto.
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.
The main antagonists could be a group of rogue ninjas who are interested in stealing Erza's magic swords. They might see her as an easy target at first, but when Naruto gets involved, they become more determined to take her down. Additionally, there could be a powerful dark mage from Erza's world who wants to bring her back and use her for evil purposes against Naruto and his friends.
In Erza's world, there could be a dark mage who wants to use the power of the unborn child for their own evil purposes. They might see the combination of Naruto's ninja abilities and Erza's magic in the child as a great source of power and will stop at nothing to get their hands on it.
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.
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.
Obviously, Naruto and Erza would interact a great deal. Naruto would also probably interact with Erza's friends from Fairy Tail. He might meet Natsu and they could have some interesting battles or friendly competitions. He could also interact with Lucy, and perhaps learn about celestial spirits from her.
Obviously, Naruto and Erza would be the main characters. Naruto is the young ninja with a lot of potential and a big heart. Erza is the strong and capable character who takes on the role of raising Naruto. There might also be other characters from Naruto's world or Erza's world that make appearances, like Naruto's friends from Konoha in cameo roles, or Erza's guildmates who interact with Naruto as he grows up.
It could be a group of rogue mages who have a vendetta against Natsu and Erza. They see the two leaving Fairy Tail as an opportunity to attack them when they are more vulnerable outside the protection of the guild.
Well, one possibility could be that Naruto, with his strong sense of justice and his own experiences of growing up alone, takes Erza under his wing. He might teach her some of his ninja skills, like how to move stealthily or use chakra in a new way. And Erza, being the strong - willed character she is, would adapt these skills into her own fighting style.
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.