Well, perhaps a character from a very low - key anime that no one expects. Like a character from a slice - of - life anime who has some hidden power. Maybe a schoolgirl who can control time in short bursts, but has been keeping it a secret in her normal life.
How about a character from an old, forgotten anime? Some sort of ancient warrior who has been sleeping for centuries and is awakened just in time for the tournament. They would bring archaic and powerful fighting techniques that no one in the modern tournament has ever seen before.
How about a group of aliens who have heard about the One Punch Man tournament and are curious to see the power levels on Earth? They come with their own set of powers, like the ability to transform their bodies into different elements. One of them might be able to turn into water, making it difficult for the heroes to land a solid punch. These aliens add a whole new level of chaos and excitement to the tournament as the heroes have to figure out how to fight against such strange and powerful opponents.
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.
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.
Captain Jack Sparrow from the Pirates of the Caribbean world. Imagine him being thrown into the Tournament of Power. His wily and unpredictable nature, along with his knack for getting out of tough situations, would make for some really interesting interactions with the more serious combatants in the tournament. He could use his cunning to outwit opponents rather than relying on pure strength.
Filch could be an unexpected character. He might enter the tournament in a comical way, like he accidentally stumbles into the selection process and ends up being a participant. Another could be Peeves. He could disrupt the tournament in his usual mischievous way, or perhaps he is made a part of a task where the competitors have to outwit him.
Orochimaru. Despite his past as a villain, he could have some knowledge or power that Naruto needs in the tournament. Maybe he has some ancient jutsu that can counter the abilities of opponents from other dimensions. His presence would also add a lot of tension within the team as some members would be very suspicious of him.
One idea could be introducing a new, underdog character who has a unique power that no one expects. For example, a character with the ability to copy and enhance the powers of those they touch. This would create an interesting dynamic in the tournament as they could turn the tables on stronger opponents.
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.
He could face Jiren again. But this time, Gohan would be the one to come out on top. Maybe he'd have studied Jiren's fighting style thoroughly and found a way to counter it.
Well, in this 'vegito tournament of power fanfiction', Vegito could form alliances with unexpected characters. For example, he might team up with a weaker fighter who has some special knowledge about the tournament's hidden rules or secrets. This could lead to some interesting plot twists. Also, his fusion might have some glitches or new developments that add drama to the story, like his power fluctuating at crucial moments.