A cool plot could be that Alice has special powers in this fanfic. She can communicate with the souls of the fallen humans in a way that no one else can. This ability leads her on a quest to find out the true history of the Underground and how to break the cycle of human - monster conflict.
One idea could be that Alice finds herself in the Underground and has to navigate through the different areas using her unique sense of wonder. Maybe she befriends some of the monsters like Toriel and they go on an adventure together to find a way out for her.
One idea could be that Alice returns to Wonderland as an adult and has to deal with new, more complex riddles and challenges that are related to the grown - up world she now knows. For example, the Queen of Hearts might be having a 'tax collection' problem in her kingdom and Alice has to use her real - world knowledge of economics to solve it.
Another plot could involve new characters being introduced into the Borderland. These new characters could have unique abilities or knowledge that either helps or hinders the main characters in their struggle to survive. Maybe they know a secret about the origin of the Borderland that could change everything.
Another plot idea is a story where the antagonists from both games, such as Flowey from Undertale and the King from Deltarune, form an alliance. Their goal could be to take over both universes. This would force the heroes of both games to work together. The fanfic could explore how the different characters with their own powers and personalities interact and strategize to defeat this powerful alliance.
One idea could be that the characters from Homestuck end up in the Underground of Undertale. They might have to deal with the various monsters there while also trying to find a way back home. Maybe John Egbert could team up with Frisk and they use their unique abilities to overcome challenges like puzzles and battles.
One idea could be that the animatronics from FNAF end up in the Undertale universe. They might be mistaken for strange monsters at first. The Sans character could be particularly intrigued by their mechanical nature and try to figure out how they work.
Your OC could be a transfer student from a far - off place with a different understanding of Alices. They might struggle to fit in at first but then use their unique perspective to solve a big problem at the school. For example, if there's a conflict between different groups of students with different Alices, the OC could find a way to bring them together through their outsider view.
One idea could be exploring how the other characters in the Bendy universe react to the news. For example, the more mischievous characters might be curious and cause some light - hearted chaos. Another plot could involve Bendy and Alice preparing for the arrival of their baby, like building a nursery in their spooky yet charming world.
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.
Perhaps in the fanfic, Alice Angel has lost her memory. Henry, who has some knowledge about the strange place they are in, tries to help her regain her memories. Along the way, they discover secrets about the place and their own pasts. Their relationship grows as they rely on each other for support, and Alice Angel starts to see Henry in a new light, different from how she might have perceived others before.
A crossover with a steampunk world would be cool. Picture Alice from 'Alice Madness Returns' arriving in a steampunk city where machinery and magic coexist. She could encounter characters like a steampunk Mad Hatter who is an inventor gone mad. The contrast between the dark, dreamy world of 'Alice Madness Returns' and the mechanical, yet fantastical, steampunk world would create interesting storylines and character interactions.