Well, it could be Orochimaru. He was always up to no good and had an interest in powerful ninja. He might target Naruto and Temari to either steal their powers or use them for his evil experiments, which would surely lead to them getting hurt.
The Akatsuki members are also likely antagonists. They were constantly on the hunt for jinchuriki like Naruto, and Temari might get caught in the crossfire while trying to protect Naruto. Their powerful and often ruthless nature would make them a great threat in the fanfiction.
One likely antagonist could be Chazz Princeton. He's often shown as a bit of a bully in the Yugioh series, and he might target Syrus out of jealousy or just to assert his dominance. Another could be an original character created by the fanfiction writer, perhaps a new student at Duel Academy who has a vendetta against Syrus for some reason. The Society of Light could also be antagonists in a sense, as they might try to manipulate Syrus or use him in their schemes, which could lead to him getting hurt.
It could be Orochimaru. He might be interested in the baby's unique genetic makeup, thinking he can use it to create more powerful experiments. He has always been obsessed with power and the baby's combination of Naruto and Temari's abilities would be very appealing to him.
It could be the same antagonists from the original Naruto series, like Orochimaru or Madara Uchiha. Since Naruto and Temari are time - traveling, they might encounter a younger version of these villains at a time when they were just starting their evil plans. Another possibility is that they create new antagonists by changing the past in some way. For example, if they prevent a certain character from dying, that character could turn out to be an antagonist in the altered timeline.
One possible antagonist could be a rogue ninja from another village. They might target Tenten for her unique weapon - handling skills, thinking they can use her to create powerful weapons for themselves. Naruto would then have to face this ninja to save Tenten.
Perhaps some rogue ninja from other villages. They might see Naruto as a symbol of Konoha's power and want to take him down to weaken Konoha. These rogue ninjas could be led by a powerful and vengeful shinobi who has a grudge against Konoha. So when Naruto is alone on a mission or something, they attack, and that's when Minato steps in to save him.
It could be some rogue ninja clans that are after Karin for her special abilities. They might see her as a tool to gain power or information. For instance, a clan that specializes in blood - related jutsu might want to use Karin's unique blood - sensing ability for their own evil purposes.
The Hidden Leaf Village itself could be an antagonist. If Naruto goes evil, the village that once was his home would be against him now.
One likely antagonist could be those who fear Naruto's new power. For example, some of the traditional shinobi clans who are used to being in power might see Naruto as a threat. Another could be ancient evil forces that were sealed away long ago and now sense Naruto's god - like power as a way to break free. They might try to either corrupt Naruto or destroy him to prevent him from stopping their plans.
One potential antagonist could be a power - hungry shinobi from another village. They might view Hayate as a threat to their plans, maybe because Hayate is involved in some sort of peace - keeping mission between villages. So, they try to eliminate him. And Naruto, being the defender of his comrades, will not stand by and let that happen. There could also be a group of missing - ninjas who band together to take on Hayate, thinking he's an easy prey, but they underestimate Naruto's loyalty and strength.
The village elders could be antagonists. They might see Naruto and Kakashi as threats to their traditional way of ruling the village and use false accusations to banish them.