One interesting story could be about the characters' reactions to different timelines. For example, Sans might react in a unique way when faced with a timeline where everyone is overly aggressive. Another could be the reactions of the monsters to human technology that they've never seen before, like a smartphone. And there could also be stories about how the characters react to fan - made scenarios, such as a 'what if' situation where Asriel didn't turn into Flowey.
Sans is one of the most commonly written characters. His laid - back yet mysterious nature makes for interesting reactions. Papyrus is also popular as his over - the - top and positive personality gives a lot of room for different reactions. Frisk is often included too, as they are the main character through which a lot of the game's story is told, so their reactions to various in - game events are frequently explored in fanfiction.
He might be shocked. In the fanfic, if he sees things like his friends' future achievements or the new threats that emerge, it would surely startle him. For example, if he discovers that one of his friends has a family of their own and has become a great leader in the village, he would be proud but also a bit surprised at how much has changed.
With his usual determination. Naruto has always been one to face difficulties head - on. So, even if he's shocked by what he sees in the fanfic's future, he'll likely try to find solutions immediately.
A common theme is the clash of cultures. Naruto characters come from a world of ninjas and shinobi values, while Undertale has a very different monster - centered society. Another theme could be the exploration of power. Naruto characters have their own set of jutsu and abilities, and when they are in the Undertale world, how these powers compare to the magic there is often explored. Friendship is also a big theme, as Naruto characters might form bonds with the Undertale monsters.
One possible plot could be that Naruto, while on a mission, somehow gets transported to the world of Undertale. He teams up with Frisk to stop Flowey from taking over both their worlds. Naruto's ninja skills, like his shadow clones, could be used in creative ways in the battles against the monsters in Undertale.
There's also 'Undertale's Influence on Naruto'. This fanfic explores how the concepts from Undertale, like mercy and determination, could impact the Naruto universe. It delves deep into the characters' psyches and how they might change with these new influences. It's a great read for those who like to see crossovers that really make you think about the characters in a new light.
A really interesting plot twist can be when the characters from Undertale end up in the Naruto world and they have to adapt to the ninja way. For example, Sans using his shortcuts in the middle of a ninja battle, which throws everyone off guard. It's unexpected and adds a whole new level of humor and chaos to the story.
One idea could be that Naruto characters end up in the Underground world of Undertale. For example, Naruto with his determination could be seen as similar to the determination trait in Undertale. He might interact with Sans and Papyrus. Naruto could use his shadow clones to confuse the enemies in the Underground. Another idea is that the ninjutsu in Naruto could be combined with the magic system in Undertale. Maybe Sasuke's Sharingan could have an interesting interaction with the soul mechanics in Undertale.