There could be a new organization that is made up of ninjas from different villages who have banded together with a dark agenda. They aim to take over all the ninja villages and rule with an iron fist. Team 7 has to overcome their combined strength and diverse abilities.
Perhaps in the fanfiction, Team 7 is called to a war between two ninja clans. They are there as peacekeepers initially, but things go wrong. Naruto tries to use his talk - no - jutsu to convince the clans to stop fighting. When that fails, Sasuke steps up and shows his combat skills to intimidate the warring sides. Sakura, on the other hand, discovers that there is an outside force manipulating the clans to fight. She then uses her medical skills to save some of the injured from both clans, which helps to ease the tension a bit as they start to realize they are being used.
The Lannisters could be the main antagonists. They have a history of being power - hungry and have caused a lot of trouble for the Starks. In this fanfiction, they might be trying to expand their territory further north, which would prompt Robb and Jon to go to war to protect their homeland.
In certain 'naruto and kurama team up fanfiction', there are antagonists from other hidden villages. They might be jealous of the power that Naruto and Kurama possess together and want to take it for themselves. So, they plot to kidnap Naruto and force Kurama out of him, which sets off the conflict between them.
In some fanfictions, there could be a group of rogue ninjas who form an alliance during the war. They have no loyalty to any particular side and are only interested in causing mayhem. Sasuke and Sakura would have to deal with their unpredictable attacks and devious plans.
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.
It could be the traditional God of War villains like the gods who oppose Izuku. Maybe Ares, if he's still around in the fanfiction's version of the God of War world, would see Izuku as a threat and try to stop him.
Another possible antagonist could be a rogue superhero who has been influenced by the chaos of the civil war and decides to turn against Steve and Wanda. This character might have their own reasons for causing trouble, like a past grudge or a thirst for power.
Some former Death Eaters who escaped justice could be the main antagonists. They might be plotting to bring back Voldemort's ideology in a more covert way. Another possibility could be new dark wizards who emerge, seeing the post - war chaos as an opportunity to gain power.
There could be an ancient evil force that awakens because of Luffy's new powers. This force was once sealed away but now wants to destroy Luffy and everything he stands for, seeing him as a disruption to the balance it once knew.
Perhaps there are demons or dark forces that are trying to disrupt the union between Bella and the God of War. These antagonists could be bent on causing chaos and destruction in their world, seeing their relationship as a symbol of hope that they want to destroy.