In some fanfictions, the main antagonist could be a powerful genjutsu user. This user forces Naruto into situations with Ino by trapping them in illusions. The genjutsu user's motives could range from simple amusement to a more sinister plot to break Naruto and Ino's spirits. Another type of antagonist could be a dark - natured spirit that latches onto Ino and Naruto has to find a way to exorcise it while also protecting Ino from its influence.
Often, it could be rogue ninjas. They might target Ino because of her family's importance in the village, and Naruto gets forced into protecting her. These rogues can be after some secret technique that Ino's family guards. Another antagonist could be a power - hungry shinobi from another village who wants to use Ino as a pawn to gain leverage over Konoha. Naruto then has to step in and stop them.
Perhaps there is a rival group or another peerage that wants to use Naruto's situation for their own gain. They might try to manipulate him into betraying his own peerage or use him as a pawn in their power struggles. They could be constantly scheming against Naruto and his new - found comrades in the peerage.
In some versions, Orochimaru could be a main antagonist. Since he has his own devious plans and is always looking for power, Naruto and Ino might have to deal with him all over again as they travel back in time. He could be trying to interfere with their attempts to change the past.
It could be the same antagonists as in the original Naruto series at first, like the Akatsuki. But if Naruto and Ino change things, new antagonists might emerge. Maybe some ninja who were once allies turn against them because they fear the changes Naruto and Ino are making to the ninja world order.
Perhaps Orochimaru, if he somehow discovers Ino's time - traveling. He has always been interested in power and forbidden jutsu. He might try to use Ino's ability for his own gain, and if she resists, he would become an antagonist. He could also try to stop her from changing things that he wants to keep as they are for his own evil purposes.
Perhaps the main antagonists are some of the old - fashioned wizarding council members who believe in arranged marriages for political stability. They force Harry into the marriage with Ginny without considering their feelings, and they would oppose any attempts by Harry and Ginny to break free from the forced union.
Sasuke could be an antagonist in a way. He might be the popular, brooding type who is jealous of Naruto's growing friendship with Ino. So he tries to create misunderstandings between them, like spreading false rumors about Naruto or trying to make Ino think Naruto is not a good friend.
Well, the main antagonists could be a group of rogue ninja who are hired by a powerful nation to take down Naruto. Since he is unkillable, they would have to use more underhanded tactics like trying to isolate him from his friends and allies. Or perhaps there is a dark force that is awakened by Naruto's unkillable state, thinking that he is a threat to the balance of the ninja world. This dark force could then become the main antagonist, trying to destroy Naruto at all costs.
In some cases, it could be the Ministry of Magic itself. If the Ministry is under the control of corrupt officials, they might force the marriage for their own political gain. They could use Harry's popularity and Ginny's family connections to further their own agendas, and thus become the antagonists that Harry and Ginny have to fight against in the fanfiction.
Maybe there are powerful dark sorcerers. These sorcerers could be trying to capture Naruto for their own evil purposes, like using his power to open a portal to a dark dimension or something, and Kurama has to protect him from them.
Another type of antagonist could be a strict professor who doesn't like Naruto's carefree attitude and Hinata's association with him. The professor might try to give them a hard time, like assigning them extra difficult projects or being overly critical in class, which becomes an obstacle in their college life and relationship.