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.
One idea could be a crossover with the Harry Potter universe. Gaster could be a mysterious wizard from a far - off land who arrives at Hogwarts with his unique brand of magic that is based on his scientific experiments in his original world. Another could be a crossover with the Star Wars galaxy, where Gaster is a brilliant scientist working on some top - secret project for the Galactic Republic. Maybe he discovers a new form of energy that has implications for both the Force and the technology in the Star Wars universe.
Obviously, Gaster himself is likely to be a main character. His mysterious nature makes him a central focus as readers are eager to learn more about him. Another potential main character could be Frisk. Their ability to reset and influence the game's world could be very interesting when combined with Gaster's enigma. They could either be allies or have some sort of conflict due to their different understandings of the world.
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.
There are fanfictions where Gaster is reunited with the other characters in unexpected ways. For example, a story could have him reappear during a crucial moment in the Underground's history and use his knowledge to change the fate of everyone. It could be really exciting as it might bring new plot twists and character developments.
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.
One idea could be that the characters from Undertale end up in the FNAF world. For example, Frisk could be trying to find a way to escape the haunted animatronics while also befriending some of them in a similar way they befriend monsters in Undertale. Sans could use his shortcuts to outwit the animatronics' movements. Another idea is to have the FNAF animatronics transported to the Underground. They might be seen as strange new 'monsters' and the Underground residents would have to figure out how to deal with them.
One idea could be that the ponies from MLP end up in the Underground world of Undertale. Maybe Twilight Sparkle could use her magic to interact with the magical elements in Undertale, like the SOULs. Another idea is to have Sans and Papyrus meet Fluttershy. Papyrus would be all excited to show off his 'coolness' while Fluttershy might be a bit shy at first but then they could become friends over their love for cooking (Papyrus loves making spaghetti).