It could be a rogue ninja who was once part of a minor group during the war but has now risen to power. Maybe they felt overlooked or mistreated during the war and now seek revenge on the major villages.
Perhaps an ancient evil that was awakened by the chaos of the war. This entity could be something that was sealed away long ago and the destruction of the war broke the seals. It would be extremely powerful and Naruto and his friends would have to figure out how to defeat it using both their ninja skills and some ancient knowledge.
Madara Uchiha might also be a main antagonist. His goals of reshaping the ninja world could be directly opposed to what Naruto is trying to achieve with his newfound 'godlike' powers. Madara would likely see Naruto as a major obstacle in his grand plan for the shinobi world.
The fanfic could also focus on the personal lives of the characters. Naruto might finally have time to focus on his relationship with Hinata. There could be stories about how they build their family, and how Naruto balances his role as a family man with his duties as a hero of the ninja world. At the same time, other characters like Sasuke could also be exploring their new identities in a post - war world, trying to find their place and purpose.
There could be a mysterious figure from Naruino's past during the war. This person has a grudge against Naruino and resurfaces in the post - war period to seek revenge, creating all sorts of problems for Naruino.
It could be an original character created specifically for the fanfic. This character could be a young ninja with a unique power or backstory, like a shinobi who has the ability to control plants in combat.
It could be a power - hungry shinobi from another village. This shinobi sees Naruto's exile as an opportunity to gain more power and influence in the ninja world. He might try to hunt Naruto down to prove his own strength or to steal Naruto's secrets.
Perhaps a secret organization within the Hidden Leaf that wants to take over the village. They use Naruto as a scapegoat, getting him banished so they can carry out their evil plans without interference from Naruto. They would be the main antagonists as they are the ones behind the false accusations and the push for his banishment.
Perhaps a new, unknown force. With Naruto gone, a previously hidden group with a vendetta against the ninja villages could emerge. They might blame the villages for the destruction caused during the war and see Naruto's death as the perfect time to strike, when the villages are still recovering and vulnerable.
A group of former war criminals who were pardoned but now want to take over the ninja world. They might have formed a secret organization during their time in prison or exile. They could be using new, dark jutsus that they developed in secret to try and overthrow the current leadership in the villages and establish their own rule.
It could be a rogue vampire who didn't like the way the war ended and blames Alucard and Seras. This vampire might have his own ideas about how the vampire world should be ruled and sees them as obstacles.
It could be some of the traditionalist elders in the Hidden Leaf. They might see Naruto as a threat even after the war, fearing his growing power and influence, and thus engineer his banishment. So they could act as antagonists as Naruto tries to find his place outside the village.