Danzo could be an interesting antagonist. He's a very secretive and power - hungry character. He might see a blade master as a loose end or a potential weapon that he wants to either eliminate or use for his own political gain in the Hidden Leaf Village. His underhanded methods would create a lot of conflict with a blade master who likely has a strong sense of honor.
It could be a rogue ninja who was once part of the same clan that the hidden blade belongs to. He wants the blade back to gain ultimate power and is willing to do anything to get it from Naruto.
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.
It could be a rogue ninja who was once a student of the Hiraishin master but turned evil. They might have a twisted view of the technique and use it for their own selfish and harmful purposes, like stealing important artifacts from different villages.
One potential antagonist could be a rogue ninja who also specializes in bojutsu. He might be jealous of the main character's skills and tries to undermine him at every turn. Another option is a powerful villain from the Naruto universe who sees the bojutsu master as a threat to his plans.
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.
It could be a rogue ninja who also has a strong affinity for wind release. This ninja is jealous of Naruto's new - found power and wants to take 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.