Who could be the main antagonist in a 'Naruto Fire Master' fanfiction?

A group of power - hungry ninjas from a neighboring village could be the main antagonists. They hear about Naruto's rainbow fire and believe that if they can take it from him, they can dominate all other villages. They start a series of attacks and plots against Naruto and his village, constantly trying to outwit him and steal the rainbow fire. Their leader could be a particularly cunning ninja who has his own vendetta against Naruto's village and sees the rainbow fire as the perfect weapon to achieve his revenge.

A primordial being related to fire, like a forgotten Titan of fire, could emerge from the depths of Tartarus. It could be angry that the Olympians have been ruling for so long and sees Percy's new fire powers as a threat or a tool to use against them.

It could be a rogue ninja who also has strong water - based abilities and wants to prove that he is the true master of water. He might see Naruto's newfound skills as a threat.

A rogue ninja who has mastered all the dark aspects of the elements could be a great antagonist. This ninja could use their powers to create disasters, like summoning tsunamis with water element or causing volcanic eruptions with fire element. They might have a vendetta against the Hidden Leaf Village and see Naruto as a threat to their plans because of his growing elemental powers.

It could be a rogue ninja who also has some form of magnetism - based power but uses it for evil. This ninja might be jealous of Naruto's natural talent and wants to prove that he is the stronger one.

It could be a rogue ninja who also has mastered Nen. He might be jealous of Naruto's newfound power and wants to steal it for himself.

Team Rocket from the Pokemon world could be the main antagonist. They might see Naruto as a threat to their plans of stealing powerful Pokemon. They could try to use their usual sneaky tactics on Naruto, but he would be too smart for them with his ninja training.

Perhaps an ancient evil spirit that was sealed away long ago and is now awakened by the power of Spinjitzu. This spirit wants to use Spinjitzu to destroy the ninja world, and Naruto has to stop it.

It could be a rogue ninja from a hidden village who has also mastered lightning jutsu and wants to prove himself stronger than Naruto. This ninja might have a personal vendetta against Naruto or Konoha.

It could be a rogue ninja who is also a master of illusions and wants to use his powers to take over the ninja world. He might see Naruto as a threat to his plans since Naruto is developing his illusion skills.