Maybe the main antagonist is an evil organization that has been secretly infiltrating the village. They target Sakura because of her skills and knowledge. They deceive her into believing that her village is the real enemy, causing her to betray her comrades.
Ash himself could be the main antagonist as he is the one doing the betraying.
It could be the very villagers who once looked up to Naruto. Their betrayal could turn them into antagonists as they might try to further harm Naruto to protect their own interests.
Danzo could also be the main antagonist. He is known for his underhanded methods and distrust of anyone with too much power. Since Naruto has the power of the Nine - Tails, Danzo might spread lies within Team 7 to make them think Naruto is a danger, causing them to betray him.
Maybe an ancient evil spirit that has a vendetta against Ash. The spirit could possess the aura guardian and make it turn against Ash. In this scenario, the main antagonist is not really the guardian but the spirit that controls it, and the story would be about Ash fighting this spiritual evil.
It could be a powerful and jealous Galar trainer. This trainer might see Ash as a threat to their own reputation and success in the region. They would then use their influence and charm to turn Ash's friends against him.
The main antagonist might be an organization that wants to take over the Hidden Leaf Village. They see Naruto as a major obstacle because of his strength and influence among the villagers. So, they devise a plan to make everyone in the village betray Naruto. They use mind - control techniques or spread misinformation on a large scale to achieve their goal. This way, they can weaken the village from within before making their move to take it over.
Another possibility is a power - hungry ninja within the village who wants to take advantage of Sakura's situation. They might try to manipulate her or use her as a pawn in their scheme to gain more power. Since she's been abandoned, she may be more desperate and easier to influence, making her a prime target for this type of antagonist.
It could be a rogue ninja who has a vendetta against Naruto. Since Naruto is now married to Sakura, this antagonist might target Sakura as well to get to Naruto. Maybe this rogue ninja was once defeated by Naruto and now wants to take revenge by causing harm to his new family.
Itachi himself could be seen as the main antagonist in this scenario. After all, he is the one doing the kidnapping.
There could be a group of mean - spirited fairies. They see Sakura as an intruder in their territory, which might be a beautiful garden that Sakura often visits. To drive her away, they use their fairy magic to turn her into a cat, thinking that she will be less of a threat in that form. And they keep thwarting her attempts to turn back into a human throughout the fanfic.