An interesting OC fanfiction could focus on an OC who is an outcast from their own dimension and ends up in the 'Star vs the Forces of Evil' world. They could have a different set of values and powers which would create conflicts and also opportunities for growth. Or, you could make an OC who is a detective - like figure in the magical world, trying to solve mysteries that are related to the dark forces in the show.
One idea could be creating an OC who is a hybrid of different magical species in the 'Star vs the Forces of Evil' universe. For example, part Mewman and part monster. This OC could struggle with acceptance from both sides. Another idea is an OC who has a unique magic ability that no one has seen before, like the power to control time on a small scale. This could lead to all sorts of adventures and challenges as they try to master it without causing chaos.
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 interesting idea could be exploring a darker side of Marco's character. Maybe he has some hidden powers or motives that are revealed in the fanfiction. Another idea could be his relationships with other characters in this 'evil' scenario. For example, how does his friendship with Star change when he takes on this new, darker persona.
Another interesting plot could be Meteora forming an unlikely alliance. For example, she teams up with a group of misfits who are also on a quest for something. Together, they face various challenges and in the process, Meteora learns about friendship and trust.
Another possibility is a crossover with 'Avatar: The Last Airbender'. Star could end up in the Avatar world and learn about bending. She might team up with Aang and his friends to fight against Fire Nation soldiers who have somehow gotten a hold of some of Star's magic spells and are using them for evil.
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.
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.
A great plot idea could be that Eclipsa discovers a hidden power within herself that is both a blessing and a curse. She has to figure out how to control it while also dealing with the politics of Mewni. Another could be a time - travel fanfiction where Eclipsa goes back to key moments in Mewni's history to change things for the better. And one more idea is that Eclipsa starts a school for different kinds of magic, which causes a stir among the traditional magic users in Mewni.