Who could be the main antagonist in 'sans turns into a girl fanfiction'?

It could be another monster who is jealous of Sans' new form. This monster might feel that Sans is getting special treatment or opportunities because he's now a human. So, this jealous monster tries to find a way to either turn Sans back into a skeleton against his will or make his human life miserable by causing trouble for him in the human world, like spreading false rumors about him.

It could be another monster who is jealous of Sans getting all this new - found attention as he turns into a human. This monster might try to stop the transformation process or make Sans' life difficult in his new human form.

Maybe Team Rocket could be the main antagonist. They might see Ash's transformation as an opportunity to finally capture Pikachu. Since Ash is in a more vulnerable state as a girl, they could try to use different tactics to take advantage of the situation.

It could be a new villain who is specifically targeting Gohan because of his new form. This villain might see Gohan's transformation as a sign of weakness or an opportunity to mess with the Z - fighters. Maybe they want to use Gohan's confusion and the chaos it causes among his friends and family to their advantage.

It could be someone who wants to take advantage of Minato's new female form. Maybe a villain who has a vendetta against Minato in his original form and sees this as an opportunity to further humiliate or harm him.

Another possibility is that a group of Koopa minions could be the main antagonists. They might be under the orders of Bowser, but they have their own little schemes going on. They could target Mario - turned - girl because they think she'll be an easier target in her new form, but they are in for a big surprise when she shows her strength.

The Joker could still be the main antagonist. He might find the idea of Batman being a girl extremely hilarious and try to use it to his advantage. Maybe he would try to play mind games with her, thinking that she would be more vulnerable in her new form.

The main antagonist could be a jealous classmate. This classmate has always been envious of Marco's popularity or abilities. When Marco turns into a girl, this antagonist sees it as an opportunity to make his life miserable. They might spread rumors or try to embarrass Marco in his new form.

Bowser could be the main antagonist. He might see Luigi's transformation as an opportunity to mess with Mario and the Mushroom Kingdom. He could try to use Luigi's new form to his advantage, perhaps by trying to make Luigi a spy or by using his new identity to cause chaos among the kingdom's residents.

Ganon could still be the main antagonist. His motives remain the same - to take over Hyrule. But now he might underestimate Link because she's a girl, which could be his downfall.