There's also 'Undertale - Pokémon Crossover Quest'. In this fanfic, the protagonist is on a journey that blends the mechanics of both games. For example, battles are not just about Pokémon moves but also incorporate some of the decision - making elements from Undertale, like sparing or fighting opponents in a different way.
One idea could be that the Pokemon are in the Underground world of Undertale. Maybe Pikachu could befriend Frisk and help them on their journey. The Pokemon could have unique interactions with the monsters there, like using their moves in battles against the monsters in a different way than in the Pokemon world.
One main feature is the blend of characters. You often see Pokémon taking on traits of Undertale characters or vice versa. For example, a Pikachu might have the determination of Frisk from Undertale.
One interesting crossover could be the characters from Undertale finding themselves in the Pokemon world. For example, Sans could be wandering around the Kanto region, interacting with Pokemon trainers. He might use his unique abilities in battles against Gym Leaders. Another could be Frisk trying to catch Pokemon, but with their pacifist nature, they might have a different approach to Pokemon training, like befriending them without battling.
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.
Another good one is 'MLP - Undertale: A Tale of Two Worlds'. This fanfic delves deep into how the values and cultures of both the My Little Pony and Undertale worlds could coexist or clash. It has detailed descriptions of the crossover elements, like how Fluttershy might befriend a Flowey - like character, which makes for a really engaging read.
Some of the Pokemon could take on the forms of Undertale characters or vice versa. For instance, a Gardevoir could take on the appearance of Toriel to help comfort the other characters. This would not only be a cool visual but also a way to blend the two universes on a deeper level. The personalities of the characters could also influence the behavior of the Pokemon or the transformed characters. There could be a lot of emotional moments and comical situations as a result of these integrations.
One interesting concept could be an OC that is a powerful mage in the Undertale fanfic world. They could have unique spells that interact with the game's existing magic system in unexpected ways.
One concept could be exploring the psychological state of the character who undertakes the genocide route in more depth. For example, how they gradually lose their sense of morality and what drives them to keep going despite the horror they are causing.
One idea could be exploring an alternate timeline where Sans and Papyrus have different roles. For example, Papyrus could be the more serious and powerful one while Sans is more carefree. Another idea is to write a fanfic centered around a new character who gets trapped in the Underground and has to navigate through the various regions, making unique friendships along the way. And a third idea could be a fanfic that focuses on the backstory of the monsters before they were trapped underground, showing how their society was and how it changed over time.
Some 'creep undertale fanfic' might focus on the more mysterious areas of the game world. For instance, delving deeper into the true nature of the Dark Rooms. A story could be crafted around a character exploring these areas and uncovering secrets that are so disturbing that it changes their perception of the entire Undertale universe. It could involve things like strange symbols on the walls that seem to have a life of their own and start affecting the character's mind.