Who would be Goku SSJ4's toughest opponent in a 'Tournament of Power' fanfiction?

Jiren could still be a tough opponent in the 'vegito tournament of power fanfiction'. His power level is extremely high, and he has great combat skills. Even for Vegito, defeating Jiren would be no easy feat as Jiren has proven his strength time and again in the original tournament.

The plot might involve Goku SSJ4 teaming up with some unexpected allies in the Tournament of Power. He could be using his SSJ4 powers in creative ways, like combining them with the techniques of his new allies to gain an edge over the other universes' competitors.

Loki would also be a difficult adversary. He's very cunning and could use his tricks and illusions to try and outwit Goku. Goku would need to rely on his senses and ki to see through Loki's deceptions.

I think Gaara could be a really tough opponent for Naruto in an MMA - like scenario. Gaara has great control over his sand, which could be used defensively and offensively in a fight. He's also very resilient and has a lot of experience in combat. His stoic nature would make it difficult for Naruto to read his moves and find an opening. And Gaara's determination to win is on par with Naruto's.

I think Zeno would be a really tough opponent. Zeno has the power to erase entire universes, and a Goku and Goku Black fusion, despite being extremely powerful, would have a hard time against such a being with god - like powers.

Vegeta can also be extremely powerful in these fanfictions. He's constantly striving to be stronger than Goku and might gain some new abilities or techniques for the tournament. Another character could be Jiren. In the original Tournament of Power, he was a very tough opponent, and in fanfiction, he might be even more powerful or have some different backstory that makes him a formidable force.

He would add more chaos. Since he's a powerful and unpredictable character, his actions would disrupt the normal flow of the tournament. He might target strong fighters first to gain an advantage.

He could be more cunning and strategic, constantly plotting against his opponents in the Tournament of Power.

Genos could be an unexpected opponent. He might be forced to fight Saitama due to some mind - control or a misunderstanding in the rules of the tournament. It would be really interesting to see how Saitama would handle fighting his own disciple, and how Genos would fare against his master's incredible strength.

Another strong opponent could be Hit. Hit has the ability to manipulate time with his time - skip technique. This would give him an edge against Broly. Broly's brute strength might not be enough when Hit can literally disappear and reappear at will, striking at Broly's weak points during his time - skips.