A powerful and corrupt Sound ninja elder could be the main antagonist. This elder might be jealous of the attention Naruto gets and wants to prove that the Sound ninjas are superior. He starts a series of attacks on Naruto and Konoha - related targets, which forces Naruto to take a stand and fight back. This elder could also be trying to take over the Sound Village completely and use it as a base for his own tyranny.
It could be a rogue Sound ninja who has his own agenda. Maybe he wants to start a war between Konoha and the Sound Village to gain power in the chaos. He would see Naruto as a threat to his plans and try to eliminate him.
It could be an evil shinobi who wants to disrupt the meeting between Naruto and Kushina. Maybe they know that Kushina has some special knowledge or power that she could pass on to Naruto, and they don't want that to happen.
It could be a jealous gamer who is envious of Naruko's skills and Naruto's natural talent. This antagonist might try to sabotage their progress in the game by hacking or spreading false rumors within the gaming community in the fanfic.
Maybe it's an ancient evil that has been awakened. It targets both Naruto and Konan because of their unique powers. The evil being could be trying to use their powers to break free from a long - held seal. So, Naruto and Konan have to put aside their differences and fight this powerful foe together.
Another possible antagonist could be a rogue Uchiha. One who didn't agree with Shisui's views on peace. This rogue Uchiha could see Naruto as an outsider interfering with Uchiha matters and Shisui as a traitor for teaming up with Naruto. So they set out to disrupt their partnership and cause chaos in the village.
One possible plot could be Naruto getting lost in the Sound Village territory and being mistaken for a spy. He then has to prove his innocence while also learning about the Sound ninja's way of life. This could lead to some interesting battles and cultural exchanges.
It could be a powerful alchemist who has gone rogue and wants to use Naruto's chakra for some dark alchemy purpose.
It could be someone who wants to steal Samehada for its power. Maybe a rogue ninja who has heard of Samehada's ability to absorb chakra and plans to use it to take over the ninja world.
Maybe a powerful rogue ninja who has discovered a way to steal the powers of divine entities. He sets his sights on Amaterasu and Naruto because he knows that if he can combine their powers, he will be unstoppable. He creates various traps and illusions to separate them and weaken them so that he can carry out his evil plan.
Maybe a rogue ninja who has somehow tapped into dark powers related to Kami's domain. This ninja wants to use these powers to take over the ninja world, and Naruto, with Kami's help, has to stop him. In this fanfiction, the rogue ninja could have some personal vendetta against Kami as well, which adds more depth to the story.
Orochimaru would likely be a main antagonist even if Naruto joins the Sound Village. Orochimaru has his own devious plans, and Naruto might not fully agree with them. Even though Naruto is in the village, Orochimaru could try to use Naruto for his own gain, like getting access to the Nine - Tails' power. So, Naruto would constantly be at odds with Orochimaru.