No, in the game's lore, there's no indication that Undertale has the ability to watch fanfiction. The game world is focused on its own story and mechanics, and fanfiction exists outside of that official world.
A 'undertale watch the game fanfic' might be about a meta - narrative. For example, it could be a story where the characters of Undertale are somehow aware that they are in a game and they watch the gameplay unfold. It could explore their thoughts and feelings about how they are being controlled or the decisions that the player makes in the game. It might also be a fanfic that focuses on the community aspect of watching others play Undertale, like sharing tips and reactions in a group setting while watching a stream of the game.
The main difference is the focus. While the original Undertale is an action - RPG with combat and exploration, the dating sim visual novel is centered around building relationships and dating. In the original, you might be fighting monsters, but in this version, you're trying to win their hearts. Also, the gameplay mechanics are different. Instead of battles, it's more about making choices in conversations to progress in the relationship.
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.
Another place to check could be Reddit. There are often subreddits dedicated to 'Undertale' fanfiction. You can post a request asking for 'undertale watch the game fanfic' or search through the existing posts. Some Tumblr blogs might also be a source. There are many 'Undertale' fans on Tumblr who create and share fanfics, so it's worth exploring there as well.
One common plotline could be the character trying to find glitches in the game world to escape. For example, they might search for areas where the code seems to be malfunctioning.
First, think about the characters you want to focus on. Maybe you love Sans and want to write about his daily life in Snowdin. Then, come up with a basic plot idea, like a mystery that needs to be solved in the Underground. After that, start writing an opening scene that grabs the reader's attention, like the human falling into the Underground in a more dramatic way than in the game.
Asgore is another character. In fanfiction with SI, we could see the SI trying to understand Asgore's decision to collect human souls. The SI might try to find an alternative solution, or they could be caught in the middle of the conflict between Asgore and the humans. This could bring out new aspects of Asgore's character, such as his sense of duty, his love for his people, and his internal struggle with the violent nature of his plan.
If the SI has a different personality trait, like extreme aggression compared to the mostly pacifist nature of the Undertale world. They could start fights with various monsters, causing a chain reaction. Monsters who were previously peaceful might be forced to defend themselves, leading to a shift in the power dynamics and the overall plot direction. Also, it could make some characters question their own pacifist beliefs.
In 'Undertale Visual Novels', the pacing can be quite different. Since they are centered around the narrative, they can slow down or speed up the story depending on the plot. The original game has a set rhythm of exploration, combat, and story progression. Visual novels often have more branching paths and different endings based on the choices you make during the story, which may not be as prominent in the original 'Undertale' game. Also, the visual presentation in visual novels might be more static compared to the dynamic gameplay of the original game.