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.
There could be an ancient evil spirit that has been awakened by the unique energy of the winter lemons. This spirit is bent on destruction and views Naruto as an obstacle in its path. It can possess people and use their skills against Naruto, making it a very difficult adversary to face. Naruto will have to figure out how to exorcise the spirit while also protecting the winter lemon groves and the people who depend on them.
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.
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.
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.
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 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 someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
It could be a rogue ninja who has a vendetta against Naruto and sees Sister Lemon as an easy target to get to Naruto. This ninja might have some special powers that make him a formidable foe, like the ability to control shadows in a more sinister way than Naruto's shadow clone jutsu.
Maybe a new character could be introduced as the main antagonist. This character could be from a hidden village that has been secretly observing Naruto's growth. They realize his potential as a weapon and plan to steal his power or use him to start a war against other villages.
Krampus himself could be the main antagonist. Since he is a figure known for punishing the naughty, his actions in the Naruto world might be seen as unjust. He could be going after ninjas without really understanding their motives, causing a lot of trouble for the main characters.
It could be a rogue ninja who also has somehow acquired bankai - like powers. This antagonist might be jealous of Naruto's newfound abilities and wants to prove that he is the stronger one. So he starts causing chaos in the ninja world, challenging Naruto at every turn.