Maybe it's an old enemy of the werewolf clan who resurfaces. This antagonist could be a powerful dark sorcerer who has a vendetta against the werewolves and sees Naruto's friendship with the female werewolf as an opportunity to strike at both of them.
It could be a rogue ninja who wants to capture the female werewolf for her unique powers. He might see her as a tool to gain more power in the ninja world.
It could be a powerful vampire lord who sees the hybrid as an abomination and wants to destroy them. This vampire lord might view the mixing of werewolf and vampire blood as a threat to the purity of the vampire race.
Maybe a powerful warlord who rules over the region where the winter lemons grow. He sees Naruto as a threat to his control over the valuable lemon resources and decides to eliminate him. The warlord might have an army of minions and some dark magic of his own that he uses against Naruto and his friends.
Perhaps an ancient evil spirit that was sealed away long ago in Naruto's world but gets awakened by Ruby Lemon's presence. This spirit has the ability to possess people and use their skills for its own ends. It starts to cause chaos in the ninja villages and targets Ruby Lemon and Naruto specifically, seeing them as the key to either its complete destruction or its total domination.
Maybe the main antagonist is an ancient spirit that has been awakened by Naruto's experiments with transformation. The spirit doesn't like Naruto's interference with nature's balance and decides to punish him by making him turn into a fox with the strange 'lemon' aspect. This spirit then tries to keep Naruto in this form permanently and drive him mad, so that Naruto can no longer be a threat to the spirit's plans of ruling over the ninja world.
It could be someone who also wants the power of the mask. Maybe a rogue ninja who has been researching the mask for a long time and sees Naruto as an obstacle to obtaining it.
It could be someone from a different ninja village who hears about the special jewelry that Naruto has. They send out their best agents, and Lemon could be working for them secretly. So, in this case, the main antagonist would be the leader of that ninja village who wants the jewelry at any cost, and Lemon is just a tool in their plan.
It could be the werewolf who bit Harry. Maybe the werewolf has some ulterior motives, like using Harry as a pawn in a bigger plan against the wizarding world. Or it could be some pure - blood wizard supremacists who see Harry's new werewolf status as a sign that he is now 'impure' and want to get rid of him.
Well, one possible plot could be that the female werewolf is from a hidden clan in the Naruto world. She meets Naruto during one of his missions in a forest. At first, they are wary of each other, but as they face common enemies together, they start to develop a bond. Their different natures, Naruto's positive energy and the werewolf's wild and primal instincts, create an interesting dynamic.
It could be Orochimaru. He might see Naruto's new form as an interesting experiment or a threat to his plans. He could try to capture Naruto to study her newfound female - related powers or use her in some dark jutsu.
The main antagonist could be an evil werewolf who wants to use Harry's new - found status for his own dark purposes. This werewolf could try to convince Harry to embrace the more savage side of being a werewolf and turn against his friends and the wizarding world. He might also be in league with Voldemort or other dark forces, using Harry as a pawn in a larger, more sinister plan.