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.
Another antagonist could be a group from a third, unknown world that sees the union of Naruto and Erza and their unborn child as a threat to the balance of the multiverse. They try to disrupt their relationship and prevent the child from being born to maintain the status quo of the different worlds.
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 potential antagonist could be Lucifer's father. He might not approve of the union or the baby, seeing it as a disruption to his grand plans or as something that could change Lucifer in a way he doesn't want. Another could be some of the demons from Hell who see the baby as a potential threat to their power or existence if it has some sort of unique abilities.
It could be some of the hospital's board members who might not approve of their relationship and Cameron's pregnancy while working at the hospital. They could try to make things difficult for the couple.
One potential antagonist could be a rogue ninja who also has aerokinesis but uses it for evil. He might be jealous of Naruto's natural talent and try to undermine him at every turn. Another could be a powerful sorcerer from a far - off land who has come to the Naruto universe specifically to find someone with aerokinesis to either steal their power or use them as a tool. There could also be a group of anti - ninja who see Naruto's new power as a threat to their plans of overthrowing the ninja system and they try to eliminate him.
It could be one of her close friends from the guild. Maybe Jellal, considering their complex relationship in the original story. There's already a lot of history between them, and this could be a new development in their relationship.
Sakura could still be an ally. Since Naruto is still Naruto in essence, his friends from the original Naruto story would likely support him. Sakura, with her medical skills and combat abilities, would be a great help.
Well, in this kind of fanfiction, Erza could potentially have some really cool interactions with Naruto characters. She might teach them some of her sword - fighting techniques that are unique to Fairy Tail. For example, her requip magic could be something that amazes the ninjas, as they've never seen magic like that before.
The villagers themselves could be antagonists in a way. Their fear of Naruto's 'demon' side might lead them to try and ostracize or even harm him.
One potential antagonist could be Orochimaru. He might be interested in Naruto's Mokuton powers for his own evil experiments. He could try to capture Naruto to study how he awakened the Mokuton and use that knowledge for his own gain.
There could be new villains from the cosmos. Since Naruto is the son of Galactus, other cosmic entities who oppose Galactus might target Naruto. For example, some powerful alien race that wants to get rid of any trace of Galactus' bloodline. They could send their most powerful warriors to the ninja world to take on Naruto.