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 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 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 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.
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.
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 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.
Another plot could involve Tom having to face his own family's past actions that are now affecting Star and the forces of evil. He has to make amends and find a way to stop his family's legacy from causing more harm, which would put him in a difficult position as he tries to balance his loyalty to his family and his love for Star.
Well, a plot could revolve around Marco and Star dealing with the reactions of their friends and family to the pregnancy. Some might be overjoyed, while others could be worried about the challenges ahead, like how they'll balance their duties and raising a child in a world full of magical threats.
One idea could be that Star discovers a magical gaming console in the Magic Realm. This console has games that are actually portals to different dimensions. She and Marco could team up to complete the levels in the games to save these dimensions from an evil force that is spreading through the gaming portals.
Another interesting plot could be about a character who discovers an unexpected side - effect of the 'Cleaved' event. Maybe there are new powers emerging or a hidden danger that only they can sense, and they have to convince others to take action.