One possibility for the antagonists could be a rogue group of ninjas who are after the power of the Sound Four. They might see the Sound Four as a means to gain more influence in the ninja world.
Well, it could be that Naruto has to face some powerful enemies while trying to rescue the Sound Four. Maybe they got caught by an even stronger group that wants to use their powers for evil. Naruto, being the hero he is, would use his signature jutsus like the Rasengan to fight off the bad guys and free the Sound Four.
There could be a group of rogue ninjas within the Sound village itself. These ninjas oppose the Daimyo's new policies and reforms. They want to keep the old ways, which were more about power - grabbing and using dark jutsu. So they start causing chaos within the village, trying to undermine the Daimyo's authority.
One possible antagonist could be Orochimaru. He might be interested in Kim's unique skills and gadgets for his own evil experiments. He could set a trap for her and Naruto would have to save her from Orochimaru's clutches. Another could be Drakken. He might team up with some ninja villains, like the Akatsuki members, in order to achieve his own goals. Naruto and Kim would then have to face this combined force.
Danzo could be a main antagonist. He was always paranoid about the Uchiha clan's power and was involved in many of the events that led to their downfall. In a fanfiction where Naruto is trying to save the clan, Danzo would likely oppose Naruto's efforts as he sees the Uchiha as a threat to his own power and the village's stability in his own twisted way.
One possible main antagonist could be Orochimaru. Since the Sound 4 were originally his followers, he may not be happy with Naruto leading them. He might try to take them back or undermine Naruto's leadership in various ways.
It could be Orochimaru. Since he had a connection to the Sound 5 in the original Naruto series, he might not be happy with Naruto leading them and try to undermine his leadership.
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.
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.
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.
In this fanfiction, it's possible that an ancient and powerful beast is the antagonist. The beast might have captured Pyraa to lure Naruto into a trap. Naruto will need to use all of his skills, including his Sage Mode if necessary, to defeat this beast and save Pyraa.