It could be someone from Naruto's past who has wronged him deeply. Maybe a hidden enemy from his childhood or a ninja who has a long - held grudge against his family. For example, a rogue ninja who was involved in the death of Naruto's parents in a different way than in the original story.
Another possibility is that an existing character, like Orochimaru, could be the main antagonist in a different light. In the fanfic, Orochimaru might manipulate Naruto's thirst for revenge for his own gain. He could use Naruto's anger to further his own experiments or plans for power, which would make him a very interesting and complex antagonist in the context of this fanfic.
Perhaps it's a new character created specifically for this fanfic. This antagonist could be a powerful and devious ninja who has a personal vendetta against Naruto. They might have a goal that directly conflicts with Naruto's revenge plot, like trying to take over the Hidden Leaf Village while Naruto is distracted by his own quest.
Most likely Naruto would be the main character as the title has 'naruto' in it. But it could also feature his friends like Sasuke and Sakura. They might support Naruto in his quest for revenge or be directly involved in the events that lead to the need for revenge.
It could be an old rival from Percy's past, someone who has always been jealous of Percy's abilities and has finally found a way to cause him great harm. This person would be a perfect antagonist as Percy would have a strong motivation to get revenge on them.
The Organization in the Claymore world could be the main antagonist. They already have a lot of secrets and do some morally ambiguous things. If Naruto shows up and starts changing the status quo, like teaching Claymore warriors to be more independent, the Organization might see him as a threat and try to eliminate him.
It could be a rogue ninja who was once a student of Orochimaru and has now developed his own dangerous jutsu. He wants to take over the ninja world and believes the old ways are holding everyone back.
Orochimaru could be a great antagonist. He might want to use Shenron's power for his own dark experiments. He could try to steal the dragon balls and manipulate the wishes to gain immortality or create an army of powerful, immortal ninja.
It could be someone who opposes Naruto's newfound admiration for Ganondorf. Maybe a traditionalist ninja who thinks Naruto is straying from the true ninja path by getting influenced by Ganondorf.
Lex Luthor could be a great antagonist. He's always looking for power and knowledge. If he discovers Naruto's abilities, he might try to use them for his own gain or see Naruto as a threat to his plans in Smallville.
It could be Orochimaru. He's always been interested in unique powers and abilities. The idea of X - 23's mutant abilities and Naruto's jinchuriki powers would be very appealing to him. He might try to capture them both to use in his experiments to gain ultimate power.
A new character could be created as the antagonist. For example, an Uchiha who has been corrupted by the power of the Sharingan and wants to use Naruto's special Uchiha - related abilities for his own evil purposes. This character could be jealous of Naruto's unique position within the Uchiha - related story and try to take it from him.
It could be a rogue ninja who has mastered parkour to an extreme level. He uses his skills to steal important ninja artifacts and Naruto has to stop him. This antagonist would be very fast and agile, making it difficult for Naruto to catch him using normal ninja techniques.