Well, you could write a fanfiction about a time - traveling adventure in the Undertale world. Maybe the protagonist travels back in time to prevent a certain event that led to the current state of the Underground. Or, a story where the monsters and humans form an alliance earlier than in the original story due to some extraordinary circumstances. Also, a fanfiction that delves into the relationship between different species of monsters in more depth could be really interesting.
An Undertale and My Hero Academia crossover is also a great concept. The heroes and villains in MHA could find themselves in the Underground. Characters like All Might could have interesting interactions with Asgore. And the idea of quirks in MHA and the unique abilities of the Undertale characters could be a really cool aspect to explore, like how Bakugo's explosive quirk might compare to Sans' bone attacks.
Well, an Undertale OC fanfiction could center around an OC that is a lost soul from a long - forgotten era of the Underground. This OC might have memories that are crucial to solving some mystery in the present Underground. Or you could have an OC that's a musical prodigy in the Underground, using their music to heal the wounded souls of both monsters and humans. Also, an OC who was experimented on by the Royal Scientist in the past and now has strange powers as a result.
Here are some plot ideas. First, the adult human could be a scientist. They meet Toriel and start to analyze the magic in the Underground. As they collaborate, they grow closer. Second, the human might be a storyteller. They tell Toriel tales of great heroes and adventures from the surface. Toriel is so captivated that she wants to be part of those stories with the human. Third, the human has a troubled past. Toriel, with her motherly nature, helps the human heal. Through this process, their relationship blossoms. In all these ideas, their relationship can develop in a non - sexual, deep and meaningful way, exploring themes of friendship, understanding and hope.
The concept of power dynamics is also fascinating. In a 'Undertale Gaster Crossover Fanfiction', Gaster's power, which is not fully understood in the original game, could be compared or combined with the powers of other characters. This might result in epic battles or the need for characters to cooperate in ways they never had to before in their respective stories.
Well, in 'mlp meets undertale fanfiction', there could be crossovers of characters. Picture Fluttershy befriending a shy monster in Undertale. Her kind nature could help the monster come out of its shell. Also, the magic system in MLP could blend with the combat or determination - based mechanics in Undertale. It might create new ways for the characters to overcome challenges. For instance, Twilight Sparkle's magic could be used to solve some of the puzzles in the Underground in a different way than the normal Undertale characters would.
Some undertale pregnant chara fanfiction might center around the theme of hope. Chara's pregnancy could symbolize a new beginning for the Underground. It could explore how the different monster species interact with Chara during this time. There could be stories where the monsters come together to support her, or there could be conflicts arising from different beliefs about the pregnancy and what it means for their future. It could also delve into Chara's internal struggle of whether she's ready to be a mother given her complex past.
One idea could be that the characters from Undertale find themselves in the FNAF pizzeria. Sans could use his shortcuts to avoid the animatronics' attacks. Another idea is that the FNAF animatronics end up in the Underground. Freddy might befriend Toriel, and she could try to 'fix' them. Maybe Papyrus would be terrified of the animatronics at first but then try to befriend them with his overly friendly nature.
One idea could be that the characters from Undertale find themselves in the world of Deltarune. For example, Frisk could end up in the Dark World and have to figure out how to get back home while interacting with the Deltarune characters. They might team up with Kris to fight against new threats that are a combination of both universes' villains.
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.
Character interactions are also great. Imagine Frisk from undertale meeting Pikachu. Their personalities are so different. Frisk is this kind and empathetic character, while Pikachu is energetic and curious. Their meeting could lead to all sorts of comical or heart - warming situations. Maybe Frisk tries to communicate with Pikachu in the undertale way, and Pikachu responds in its own pokemon - like way.