It could be an old enemy of the village who has been waiting for the seal to break. Maybe someone who was imprisoned by the same seal long ago and now wants revenge on the village for sealing them away.
Danzo could be a great antagonist in this fanfic. He was always power - hungry and had his own secretive ways. He might see the five pronged seal as a way to control Naruto or to use his power for his own dark purposes. He could try to manipulate Naruto or those around him to keep the seal in place or even make it stronger for his own gain.
It could be a rogue Kumo ninja who wants to steal Naruto's seal - mastering knowledge for his own evil purposes. Maybe he wants to use the seals to take over Kumo or even the whole ninja world.
Well, it could be about Naruto suddenly finding out that the broken seal has unleashed some ancient and powerful chakra within him. Maybe he has to learn to control it quickly to face a new, powerful enemy that has emerged because of the seal breaking.
A rogue shinobi who has found a way to corrupt the Reaper Death Seal and use it for his own evil purposes. He could be stealing the souls of innocent people instead of using it as a last - resort jutsu like it was intended.
It could be a new character created by the fanfic writer. Maybe a young orphan who discovers their seal - making talent by accident.
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.