One idea could be that Star starts a harem of different magical beings from across the dimensions she visits. For example, she might meet a water nymph in one dimension and a fire spirit in another, and they all form a unique bond with her. This could lead to adventures where they have to use their combined powers to face threats to the multiverse.
A possible plot for a Star vs the Forces of Evil harem fanfic could be that an ancient prophecy foretells of a harem that will save the universe. Star is at the center of it, and she has to gather different individuals with special abilities. As they come together, there are cultural clashes and power struggles within the harem, but they eventually learn to work together to defeat an ultimate evil that is threatening all of existence.
Well, perhaps a plot where Star accidentally creates a harem due to a misfired spell. The members of the harem could include her friends like Marco, but also some new and interesting characters. They could then go on a journey to reverse the spell while dealing with the comical and sometimes complex relationships that develop within the group.
One idea could be exploring Star's life if she had grown up on Mewni instead of Earth for a while. Maybe she'd have different relationships with the monsters and her royal duties.
One idea could be exploring an alternate universe where Star and Marco's roles are reversed. Another could be a fanfic that focuses on the backstory of a minor character like Pony Head. Or perhaps a story where the forces of evil have a more sympathetic origin.
One idea could be that Ben and Star team up to stop an interdimensional villain who is stealing magical and alien powers. They could combine the Omnitrix's abilities with Star's magic.
A possible plot could involve the moon and star being from different realms. The forces of evil are trying to merge these realms in a chaotic way. The moon and star must figure out how to stop this while also learning about each other's powers and cultures. They could start off not trusting each other fully but as they face more and more challenges from the forces of evil, they grow closer and become a more powerful duo.
One idea could be an OC who is a time - traveler from a parallel dimension where the forces of evil have already won. This OC tries to warn Star and her friends and helps them change the course of events. Another plot could involve an OC with a unique magical ability that neither Star nor the forces of evil have seen before, and this ability becomes crucial in a major battle. Or an OC who is secretly related to one of the main characters and their discovery of this relation causes a stir in the story.
One idea could be that Star and Marco are dared to swap bodies for a day. It would lead to a lot of comical situations as they try to adjust to each other's lives. Another could be that they are dared to reveal their deepest secrets about their feelings for each other, which could potentially change the course of their relationship.
Well, a cool plot might involve a mermaid princess in the Star vs the Forces of Evil world who is in love with Marco. Star helps her to win Marco's heart in exchange for some magical mermaid artifact that can help Star in her fight against the forces of evil. Or perhaps there's a mermaid coven that has been cursed by Ludo's family long ago, and Star has to break the curse to gain their allegiance against the ever - growing forces of evil.
Perhaps in the fanfiction, Toffee's memories are transferred into Star's mind. Star then has to deal with not only her own thoughts but also Toffee's memories and emotions. This could lead to her making decisions that are influenced by Toffee, which confuses her friends and family. She has to find a way to separate Toffee's memories from her own without losing her own identity in the process.
Perhaps a plot where the forces of evil from both universes join together. Ben and Star have to form an alliance with their respective friends and family. They must learn about each other's powers and strategies to defeat this combined evil. Ben might teach Star about his alien tech, and Star could show Ben how to use magic in combat.
One idea could be that Eclipsa discovers a new, hidden power within herself that gives her an edge against the forces of evil. Maybe it's a long - lost spell from her ancient lineage. Another could be a team - up between Eclipsa and some unexpected allies from different realms to fight the forces of evil.