Perhaps a new, powerful and mysterious ninja who has a vendetta against the Hidden Leaf Village where Sakura is from. He sees Zetsu as an accomplice of the village and targets both of them, causing chaos and danger in their lives. This new character could bring in a fresh and unexpected element of danger to the fanfic.
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.
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.
It could be a rogue ninja who has a vendetta against Naruto and wants to target his family. Maybe this ninja was once defeated by Naruto and now wants to take revenge by harming his loved ones, especially his and Sakura's child.
Itachi himself could be seen as the main antagonist in this scenario as he is the one performing the kidnapping. His actions are against Sakura's will and are likely to cause distress and conflict.
There could be an old - fashioned lemonade tycoon as the antagonist. He has a monopoly on lemonade sales in the area and sees Botan's new and innovative lemonade recipes as a threat to his business. So, he uses his wealth and influence to try and drive Botan out of the lemonade - making business, creating all sorts of obstacles for her along the way.
It could be a rival cowboy named Orochimaru. He's jealous of Sasuke's skills and the fact that Sakura is with him. Orochimaru might try to steal Sasuke's horse or sabotage their quests for supplies. He could also spread false rumors about them in the small Western towns they pass through, causing trouble for them.
Mr. McGregor could be the main antagonist. He might be jealous of the special lemons that Peter Rabbit has found and try to take them away. He could see the lemons as something valuable for his own use, like making a delicious and profitable lemonade.