A new character could be introduced as the antagonist. For example, a powerful wizard from a far - off land who wants to use Princess Peach's magic during Showtime for his own evil purposes. He could be a complex character with his own motives and backstory, adding a fresh twist to the fanfiction.
Bowser could be the main antagonist as usual. He might be jealous of Princess Peach's newfound popularity during the Showtime and decides to crash the event and kidnap her once again, leading to a rescue mission by Mario and his friends.
Perhaps the main antagonist could be an alternate version of Princess Peach herself. Maybe from a parallel universe where she turned evil. This evil Peach could try to take over the real Peach's kingdom during the Showtime, causing chaos and forcing the real Peach to face her own dark side in order to defeat her.
One idea could be that Princess Peach discovers a hidden power within herself during the Showtime event that allows her to communicate with the spirits of the Mushroom Kingdom's past rulers. She uses this newfound ability to learn ancient secrets and protect the kingdom from a new, unexpected threat.
The main antagonist could be an evil knight who has a vendetta against dragons. When the princess turns into a dragon, he sees her as a prime target and tries to slay her, believing all dragons are evil. He doesn't care that the princess was once human and is only focused on his own prejudiced views.
It could be an envious noble. This noble might be jealous of Sakura's position as a princess and her popularity among the people. So, they try to spread false rumors about her and plot to dethrone her.
It could be a rival trainer who is jealous of Ash's success and also wants to use Princess Sara for her influence. He spreads false rumors about Ash to turn the people of the princess's kingdom against him. He then tries to take over the role of the hero in the situation, but his real intentions are to gain power and wealth by any means necessary.
It could be a power - hungry warlord who wants to conquer both Naruto's land and Princess Kuyoki's kingdom. He sees their growing alliance as a threat and tries to stop them at every turn.
It could be a dark wizard or sorcerer who was the one responsible for the princess's death in the first place and doesn't want her to be resurrected. They might have their own dark motives like wanting to rule the princess's kingdom or gain some powerful magic from her death.
A power - hungry nobleman who wants to take over the throne from the princess could be a great antagonist. He might try to undermine her at every turn, spreading false rumors about her and trying to gain the support of the people.
A powerful wizard who is conducting some strange experiments. He thinks that by forcing a boy to be a princess, he can create some sort of magical balance in the world. He doesn't care about the boy's feelings or rights and just uses his magic to enforce this transformation.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
Miss Minchin could be an obvious choice as the main antagonist. She's already mean to Sara in the original story, and in a fanfic, her meanness could be intensified. For example, she could try to prevent Sara from having any friends or getting an education.