A really cool plot twist could be that Papyrus watches the fight and discovers a hidden power within Sans. As he watches, he notices something different about Sans' attacks or defenses. This leads to Papyrus helping Sans to fully unlock and control this newfound power.
Another plot twist could be that Papyrus realizes the fight is all an illusion. In the fanfiction, he might see through some sort of magic or mind - control that was making Sans fight. So, he then has to figure out how to break the illusion and save Sans from this false battle.
Papyrus might be initially shocked. He's used to Sans being more laid - back, so seeing him in a fight could be quite a surprise. His eyes would widen and he might gasp.
One cool plot twist could be that in one of the multiverse scenarios, their enemies are actually their allies. This would completely turn their understanding of good and evil on its head and force them to reevaluate their relationships. It could lead to some really intense character development as they try to figure out what this means for their own world.
One interesting plot twist could be when a character from Burgess who was thought to be just an ordinary person turns out to have a hidden power that is crucial in helping the Guardians. For example, a young girl might discover she can control ice like Jack Frost, which was unexpected and changes the whole dynamic of their fight against the bad guys.
Well, it could be about Gaster suddenly reappearing in the lives of Sans and Papyrus. Maybe he has some important knowledge or a mission for them. For example, he might need their help to fix something in the Underground that's going wrong.
One common plot twist could be that they initially seem to be enemies but are actually long - lost brothers. Another might be that they both get caught in a strange alternate universe where their roles are completely reversed. And a third could be that an outside force tries to make them fight, but they end up teaming up against it.
One interesting plot could be their initial confusion when they meet. Sans is used to his own world's Papyrus, and suddenly meeting Underswap Papyrus would be quite a shock. They might have a comical exchange trying to figure out who the other is.
One possible plot twist could be that Zeref actually doesn't want to fight Natsu seriously and has some hidden agenda. Another could be that Natsu discovers a new power during the fight that is related to his past or his true identity.
Often, there's a plot twist where the Criminal Underfell Sans reveals a hidden side of himself that is not entirely bad. He might have been forced into a life of crime due to certain circumstances. And this revelation makes the Swap Cop Sans see him in a new light. This could lead to a change in the power dynamic between them and also the direction of the story as a whole.
A plotline could be that they discover a hidden part of the Underground. This new area could have different rules and strange creatures. They have to adapt quickly to survive and also figure out how this new place relates to the rest of the Underground.
A great idea could be a story about their daily adventures in the Underground. For example, they could be on a quest to find a rare ingredient for a new recipe Papyrus wants to try. This would focus on their brotherly bond and Papyrus's enthusiasm for cooking, while Sans tags along with his laid - back attitude.