Obviously, Kakuzu could be the main antagonist as he is the one using the tendrils to wrap around Sakura. His motives for doing so, whether it's for power, revenge or some other reason, would make him the force that Sakura has to oppose.
Well, it could be about Kakuzu using his tendrils to capture Sakura for some unknown and perhaps dark purpose. Maybe he's under the control of an enemy who wants Sakura for her unique abilities. Or it could be a more complex story where the tendrils are not just for capturing but also for some sort of connection between Kakuzu and Sakura, like a strange form of communication.
She might get injured. The black tendrils could be very strong and sharp, and if Sakura is not careful, they could cut her or pierce through her defenses.
In fanfiction, Sakura might struggle against Kakuzu's black tendrils due to their menacing nature. Kakuzu is a powerful foe, and his tendrils could represent a significant threat to Sakura. She could use her medical ninjutsu skills to try and counteract any negative effects the tendrils may have on her or her comrades.
Based on the title, Sakura is likely to be one of the main characters. As for others, it's hard to say without reading it. There could be characters related to the kakazu tendrils, perhaps the ones who control them or are affected by them.
In this fanfiction, the main antagonist could be a strict teacher. Sakura, being a boy with a free - spirited nature, often clashes with the teacher's rigid rules. The teacher always seems to be out to get Sakura, giving him unfair punishments and trying to suppress his creativity or individuality.
It could be a powerful dark wizard who wants to use Sakura's powers for his own evil purposes. He might be constantly trying to capture her or manipulate her into doing his bidding.
Another potential antagonist could be an enemy that Orochimaru and Sakura face together during her training. This enemy might have a vendetta against Orochimaru and see Sakura as an extension of him. So while Orochimaru is training Sakura, this new enemy could be the one causing the most immediate problems and danger for them both.
Another option is a jealous individual from Sakura's own time who somehow manages to follow her into the past. This person is envious of Sakura and wants to take over her role in the past - time events. They would constantly try to undermine Sakura and take credit for her actions, causing a lot of trouble for her during her time - traveling adventure.
A rogue ninja who wants to steal the power of the contract for his own evil purposes could be the main antagonist. He might be jealous of the connection between Sakura and Itachi and wants to break it to gain the abilities they could potentially unlock together.
Maybe an ancient evil spirit that has been awakened by the bloodshed of the Warring States period. It starts to possess the warlords and turn them against each other, and Sakura has to find a way to exorcise the spirit before it destroys everything. In this case, the spirit is the main antagonist as it is the root cause of the chaos.