Who could be the main antagonist in the 'Naruto Master of Water Fanfiction'?

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.

Perhaps an ancient demon that was once sealed away but has now been released due to some unforeseen circumstances. The demon has an affinity for fire and sees the Fire Master as a threat to its plans of world domination. It would constantly try to eliminate the Fire Master and anyone who stands in its way, including Naruto and his friends.

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.

Perhaps an old enemy from Naruto's past who has resurfaced with new and enhanced combat abilities. This enemy holds a grudge against Naruto and the Leaf Village and aims to destroy them both. He has studied Naruto's combat style and found ways to counter it, making him a formidable antagonist.

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.