A plot could involve a love story between a character from Undertale and one from Underfell. Their different worlds' values and cultures clash, but they still manage to find common ground and fall in love. This could lead to them trying to bridge the gap between the two universes to be together.
One interesting plotline could be the clash of different character personalities. For example, the more aggressive characters from Underfell interacting with the kinder ones from Undertale. They might start off not getting along but gradually learn from each other.
Maybe in the fanfiction, the Dark Fountains from Deltarune start appearing in the Undertale world. This causes chaos as the monsters in Undertale don't understand what these strange fountains are. Frisk, along with the Deltarune trio, has to figure out how to stop the fountains from spreading darkness. They could learn from each other's powers and strategies. For example, Frisk could teach Kris about determination in a more in - depth way, and Kris could show Frisk how to navigate the strange new areas created by the Dark Fountains.
Well, a cool plot for 'Undertale meets Underswap fanfiction' might involve a character exchange program of sorts. Papyrus from Undertale goes to Underswap and tries to teach his Underswap counterpart how to be more confident in his cooking skills. At the same time, the Underswap Sans could come to the normal Undertale world and introduce new ways of having fun to the rather serious Sans there. This could lead to a series of comical and heartwarming interactions between the characters from both universes.
One interesting plot idea could be that characters from Undertale end up in the Overtale universe and have to adapt to its different rules and power systems. For example, Sans might find it strange that magic works in a more complex way in Overtale. Another could be a character from Overtale getting lost in the Undertale world and having to deal with the moral choices that the Undertale characters face, like whether to kill or spare enemies.
One idea could be that the characters from Undertale find themselves in the Deltarune world and have to adapt to its different rules and characters. For example, Sans could meet Ralsei and they could have a humorous exchange about the nature of their respective universes.
Sans is often a very popular character in these fanfictions. In both Undertale and Underfell, he has a distinct personality. In Underfell, his more edgy and somewhat darker side can create interesting interactions when combined with the regular Undertale Sans.
In such a fanfiction, there could be a lot of conflict. Underfell Sans is from a more hostile AU than Undertale. He might initially be distrustful when meeting the characters from Undertale. For example, he could misunderstand their kindness as weakness or a trick.
Frisk would be a likely candidate. Since Frisk is the main character in Undertale and has a very adaptable personality. They could approach Underfell Sans with an open mind and try to understand him.
One idea could be a story where Fell Sans and Fell Toriel form an unlikely alliance. Maybe they start by just having chance encounters in the Underground, and over time, they realize they share a common goal, like trying to bring some sense of order to the chaotic Underfell world. This could involve them working together to deal with unruly monsters and gradually growing to respect each other.
Frisk is also quite popular. Their role as the main character in Undertale and how they would interact with the darker or different versions of characters in Underfell and the swapped - up world of Underswap makes for great fanfiction material. They can be the mediator or the catalyst for change in the crossover story.