Perhaps a time - traveling gamer from the future. He comes back to the present in the 'Star vs the Forces of Evil' universe to stop a glitch in the space - time continuum that is being caused by a malfunctioning magical game. He teams up with Star and her gang, bringing with him advanced gaming and time - travel technology.
Obviously, Star could be one of the main characters. She could have an adventure in the mermaid world and her character traits like her fearlessness and her ability to adapt would make for an interesting story. Another main character could be a mermaid named Marina. She could be the one who helps Star understand the mermaid world and they form a strong friendship.
One common theme is the adaptation of the characters' powers to the gaming world. For example, Star's magic wand might work differently in a game setting.
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.
There's a fanfiction called 'Gaming with the Forces of Evil'. In this one, Star and her friends discover a new video game that seems to be connected to their real - world magic. It's a really interesting concept and the author does a great job of mixing the two elements together. The characters' personalities are well - maintained from the original show, which makes it even more engaging.
Well, obviously Tom and Star are the main ones. Tom with his demon powers and complex personality, and Star with her magical princess powers and her free - spirited nature. They are the core of any Tom Star fanfiction.
Well, obviously the moon and star themselves could be the main characters. They could be personified entities with their own personalities. The moon could be calm and wise, while the star could be more impulsive and energetic.
Well, from the 'Star vs the Forces of Evil' universe, obviously Star and Marco are must - includes. They are the main characters and their dynamic is really interesting. In a crossover with 'Gravity Falls', Dipper and Mabel would be great additions. Dipper's curiosity and Mabel's wackiness could create some really fun interactions with Star and Marco. They could all investigate some strange phenomena that combines the mysteries of both Gravity Falls and Star's dimension.
It could be an ancient, powerful demon that has been awakened by the chaos in the magical realms. This demon might have a vendetta against Eclipsa's family specifically and wants to destroy her line.
Maybe the main villain is a corrupted ninja who once was on the side of good but was seduced by the power of the dark side. He now leads the forces of evil and has a personal vendetta against Naruto Star, seeing him as a threat to his newfound power and plans.
A powerful dark wizard from another dimension could serve as the main antagonist. He might be interested in Meteora's unique powers and wants to use them for his own evil purposes. So he constantly tries to capture or manipulate her throughout the fanfiction.