It could be someone who is jealous of Naruto getting the monkey contract. Maybe a rogue ninja who wants the power for himself.
In this fanfiction, the main antagonist could be a hidden clan that has a long - standing feud with the monkeys or the originators of the monkey contract. They see Naruto getting the contract as a threat to their hidden agendas. They would do everything in their power to stop Naruto from fully utilizing the contract, whether it's through direct attacks, setting up traps, or trying to turn Naruto's friends against him.
Perhaps Orochimaru. He is always after new sources of power. Seeing Naruto with the monkey contract would be a great temptation for him. He might try to either steal the contract or find a way to control Naruto to use the contract's power for his own evil plans.
Maybe it's an ancient evil that was sealed away long ago and the colossus contract is the key to its release. This evil force would be the main antagonist as it tries to break free and cause chaos in the ninja world. It could be something like a powerful demon that was once defeated by the first Hokage but is now on the verge of coming back because of the colossus contract. Naruto would have to stop it with the power of the colossus.
It could be Sasuke. I know they are friends in the original, but in this fanfiction, Sasuke might turn against Naruto for some reason. Maybe he becomes jealous of Naruto's newfound power or status.
A new and powerful ninja from a far - off land could be the main antagonist. This ninja has heard of Benihime's power and wants it for himself. He sees Naruto as an easy target at first but soon realizes that Naruto is not going to give up Benihime without a fight. This antagonist could have unique powers that are a challenge for Naruto to overcome while protecting Benihime.
The main antagonist could be an ancient snake spirit that was bound to the contract. This spirit might not like the idea of Naruto, a human, having control over the powers of the contract. So it tries to possess Naruto or drive him insane by constantly whispering dark thoughts and trying to make him use the snake powers for evil. It would be a battle of wills between Naruto and this malevolent spirit.
In the 'naruto gets the monkey contract' fanfiction, Naruto could experience a whole new level of power and adventure. Firstly, the monkeys he summons could have different forms and powers. Some might be great at stealth, while others could be powerful in brute strength. This would give Naruto more options in battles. Secondly, the monkey contract might be related to some ancient ninja secrets or prophecies. Naruto could be the key to unlocking these secrets, which would attract the attention of both friends and foes. And finally, his bond with the monkeys could be a heart - warming aspect of the story, showing how he can form connections with different beings in the ninja world.
Sasuke could potentially be an antagonist. With Naruto having the Rinnegan, Sasuke might see him as a threat to his own power and goals. This could lead to a new rivalry between them, different from their previous one.
Well, it could mean he gains new powers related to the monkeys. Maybe he can summon really powerful monkey-like creatures to help him in battles. For example, he might be able to call on monkeys with unique jutsu or abilities that normal summons don't have.
Maybe an old enemy who was thought to be defeated but has somehow returned stronger. Orochimaru could be a possibility. He was always obsessed with power and immortality. If Naruto is revived, Orochimaru might see it as an opportunity to study Naruto's new state of being (since he was revived) and use that knowledge for his own dark purposes.
The main antagonist might be an organization that wants to take over the Hidden Leaf Village. They see Naruto as a major obstacle because of his strength and influence among the villagers. So, they devise a plan to make everyone in the village betray Naruto. They use mind - control techniques or spread misinformation on a large scale to achieve their goal. This way, they can weaken the village from within before making their move to take it over.
It could be a long - lost relative of an old enemy. For example, if there was a powerful dark wizard that Natsu defeated in the past, this relative might come seeking revenge and be powerful enough to kill Natsu.