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.
A group of hunters could be the main antagonists. They might be after Pitch Black because they sense that he's not an ordinary cat. They could have some connection to the dark forces that Pitch Black was originally associated with. They want to capture him and use his powers for their own gain, not caring about what will happen to Pitch Black in the process.
Well, perhaps Sakura is a girl who has some sort of magic spell cast on her. She could be turned into a cat as a punishment or by accident. Then she has to figure out how to break the spell while dealing with the life of a cat, like climbing trees and chasing mice.
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.
It could be a powerful dark wizard or sorcerer who has a grudge against angewomon or wants to use her power in some way, so they turn her into a keychain.
It could be a villain who wants to steal all the cookies in the world. This villain sees Android 18's new - found love for cookies as a threat to his plan, so he tries to stop her from spreading the joy of cookies.
A powerful and jealous digimon who has always been envious of angewomon's status and power could be the main antagonist. They might have found a way to turn her into a keychain to get rid of her.
Perhaps the main antagonist could be a dark magic user who was the one responsible for turning Harry into a girl in the first place. Their goal might be to disrupt Harry's life and his role in the fight against the dark forces. This antagonist could continue to create obstacles for Harry as a girl, like trying to prevent him from getting the help he needs to change back or causing trouble for his friends who are trying to support him.
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.