Perhaps a dark sorcerer. The sorcerer might be after Father Lemon's special knowledge or Sansa's claim to the throne. He would use dark magic to try and stop them from achieving their goals and causing chaos in the kingdom.
It could be a power - hungry nobleman who sees Sansa and Father Lemon's growing influence as a threat. This nobleman might try to undermine them at every turn, spreading false rumors and plotting against them.
There could be an old - fashioned lemonade tycoon as the antagonist. He has a monopoly on lemonade sales in the area and sees Botan's new and innovative lemonade recipes as a threat to his business. So, he uses his wealth and influence to try and drive Botan out of the lemonade - making business, creating all sorts of obstacles for her along the way.
A big, mean - tempered wild boar could be the main antagonist. It might be territorial and constantly try to chase Bambi and his father out of certain areas of the forest.
Mr. McGregor could be the main antagonist. He might be jealous of the special lemons that Peter Rabbit has found and try to take them away. He could see the lemons as something valuable for his own use, like making a delicious and profitable lemonade.
It could be a super - powered being who has the ability to control lemons and make them do his bidding. He sees Elastigirl as a threat to his lemon - based empire and tries to take her out. He might have a gang of lemon - themed minions to help him in his evil deeds. He could be someone who was once wronged by the city and now wants revenge using lemons as his weapon of choice.
Perhaps a new, powerful and mysterious ninja who has a vendetta against the Hidden Leaf Village where Sakura is from. He sees Zetsu as an accomplice of the village and targets both of them, causing chaos and danger in their lives. This new character could bring in a fresh and unexpected element of danger to the fanfic.
It could be an evil sorcerer. This sorcerer might be after Noctis because of his special powers. He sees Noctis as a threat to his plan of ruling the world. The sorcerer could also target Iris because she has a pure heart, which he wants to corrupt, and Lemon because Lemon has a map that leads to a powerful magical artifact that the sorcerer desires.
A powerful crime lord could be the main antagonist. If Castle's father was getting too close to uncovering their illegal activities, they might have made him disappear. Now that Castle is hunting for his father, they try to stop him by any means necessary, sending their henchmen after him and trying to mislead him with false information.
A powerful water nymph could be the main antagonist. Maybe she's jealous of the special bond between Percy and Poseidon and the power of the lemon. So she tries to steal the lemon and use its power to take over Poseidon's domain.
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.
Well, maybe Father Lemon has a special gift, like the ability to communicate with animals. Sansa, being intrigued, teams up with him. They use this gift to gain an edge in battles or to solve mysteries in the kingdom. Or it could be that Father Lemon has a hidden past related to Sansa's family. As they explore this connection, they form a strong bond and face many challenges together.