Perhaps there are some powerful dark forces in Salem's world that do not want Naruto to interfere. These dark forces could be the main antagonists. They might see Naruto as a threat to their existence or their way of ruling. They could try to use Salem's power against Naruto or manipulate the situation to get rid of him.
Another possibility is that there could be a new character created specifically for this fanfiction. This character could be jealous of Naruto's influence on Salem or the people in her world. So, this new character would try to undermine Naruto's efforts and become the main antagonist.
It could be Salem herself if the fanfiction focuses on Naruto trying to stop her evil plans. Since Salem has a long history of causing chaos and destruction, Naruto would see her as a force to be reckoned with and try to oppose her.