Another possibility is that the main antagonist is a former ally of Naruto who has been corrupted by power. This person was once good but something happened to them, and now they see the tournament as a way to gain ultimate power. They use their knowledge of Naruto and his friends to set up difficult challenges in the tournament, trying to break Naruto's spirit and defeat him.
It could be a powerful rogue ninja who has organized the tournament for his own evil purposes. Maybe he wants to gather all the strong ninjas in one place to defeat them and prove his superiority.
Well, it could be about Naruto entering a secret and dark tournament where the rules are brutal. Maybe he has to fight against powerful foes he's never encountered before, like some rogue ninjas with new and dangerous jutsus. And there could be some mystery behind the tournament, like who's really organizing it and what their end goal is.
Maybe the Akatsuki as a whole. They are a group of powerful and dangerous ninjas. The Black Knight's presence might disrupt their plans to collect all the tailed beasts. They would see him as an obstacle and try to eliminate him. For example, Pain might think the Black Knight is a threat to his vision of peace through pain and send his subordinates to fight him.
In some fanfictions, Goku Black has been depicted as having a hidden power reserve. In the Tournament of Power, he could tap into this reserve and achieve a new level of transformation. This new form would give him increased speed, strength, and energy projection. He might use this form to take on multiple strong opponents at once, like the powerful warriors from Universe 7 or other universes. His attacks could become more unpredictable, such as his energy beams changing direction mid - flight to catch his foes off - guard.
It could be an unknown ninja who has been researching dark and forbidden arts. This ninja accidentally unleashes the Black Death - like plague while trying to gain ultimate power.
A powerful warlord who has his own private army could be the main antagonist. He wants to take over the area where the Black Hawk Down events are taking place and will stop at nothing to achieve his goal. Naruto and the soldiers would have to work together to defeat him.
Iruka. He's mostly seen as a teacher and not a combat - heavy character. But in fanfiction, he could be given some hidden powers or a special training arc before the tournament. His inclusion would be unexpected because everyone underestimates him, but he could use his knowledge of Naruto and the other ninja to come up with unique strategies in the tournament.
It could be a new character. Maybe a powerful ninja from a far - off land who has heard rumors of Naruto's strength and is determined to draw it out and defeat him to prove his own superiority. This antagonist might be secretly spying on Naruto and trying to find ways to force him to show his true power.
In fanfiction, Goku Black might have some unique power - ups during the Tournament of Power. He could tap into a new form that's even more menacing than his regular state. This would give him an edge over his opponents and make for some exciting battles. He might also use his wits and manipulation skills to turn the other fighters against each other.
An organization of power - hungry individuals who are experimenting with the Black Fire Release to create super - weapons. They would stop at nothing to achieve their goals, even if it means sacrificing countless lives. Their leader, a cold - hearted and calculating figure, would be the main antagonist driving the organization's actions.
Hit would also make for a fascinating pairing. Hit is a skilled assassin with time - manipulation abilities. He could be a great foil to Goku Black. Maybe Goku Black tries to outwit Hit in his time - stop techniques, or they could form a temporary alliance based on their mutual desire to eliminate certain opponents in the tournament. There would be a lot of cat - and - mouse action between them.