A group of fanatical hunters who believe that all dark entities must be eliminated could also be antagonists. They would hunt the 'lady of darkness' without understanding her true nature, causing a lot of conflict in the story.
A powerful wizard who wants to use the 'lady of darkness' powers for his own evil gains could be an antagonist. He might try to manipulate her into doing his bidding.
A rival law firm could be the antagonists. They might try to steal Joe's clients or undermine his cases in unethical ways, like spreading false rumors about his legal abilities.
A group of rogue ninjas who have made the island their base could be antagonists. They might see Naruto as a threat to their operations on the island.
The Ministry of Magic could be antagonists. If they start to oppress those with different views, including Harry, it could drive him towards darkness. Their corrupt and unjust actions might make Harry think that the only way to fight back is by using dark magic.
A group of traditionalists in Akeno's world could be antagonists. They might not approve of the idea of a harem or of an outsider like Naruto having so much influence. So they could try to break up the harem and drive Naruto out of their world by using political means, like spreading false rumors about Naruto and his friends.
In Percy Jackson father fanfiction, the sea god Triton could be an antagonist. He might be jealous of Percy getting so much attention from Poseidon. The primordial sea gods or goddesses could also be antagonists. They might see Poseidon's relationship with a half - blood as a perversion of the natural order. And then there are always the minor gods who might feel that Percy and Poseidon are getting too much power and try to undermine them.
There could be a dark spirit that haunts the woods. It's jealous of Bella's presence and tries to drive her out. Another antagonist could be a neighboring landowner who wants to expand their property into the woods where Bella lives.
If it's a crossover with the One Piece world, Blackbeard could be a potential bad guy. His ability to steal Devil Fruit powers and his ambition to become Pirate King could lead to a clash with Gohan. Gohan's pure - hearted nature would be in opposition to Blackbeard's more devious ways, and their battle could be epic, with Gohan's ki powers against Blackbeard's dark powers.
Perhaps a powerful and corrupt merchant group could be the antagonists. They are secretly trying to take over the village's economy and resources. They use bribes and blackmail to get their way. The Sixth Hokage discovers their plot and has to stop them without causing too much chaos in the village's economic system. They may also have some ninja mercenaries working for them.
One potential antagonist could be a jealous sorcerer. This sorcerer sees the special bond between Pearl and the mystery girl and wants to break it. He might try to use dark magic to turn the mystery girl against Pearl or steal her powers if she has any.
Salem herself could be a great antagonist. She might be after the Summer Maiden's powers to add to her own strength or use them to achieve some evil plan. Another option could be a new character created specifically for the fanfiction, like a rogue huntsman who has turned to the dark side and wants the Maiden's power for his own gain.