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.
Perhaps a corrupted version of one of Meteora's friends. They could have been influenced by a dark force and turned against her. This would add an emotional layer to the story as Meteora would have to fight someone she once cared about.
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.
It could be one of Toffee's old allies who was in the shadows before. Maybe someone who was just as power - hungry as Toffee but was waiting for the right moment to strike. Another option could be a corrupted form of a character we already know, like a dark version of Marco. He could be turned against Star by some dark magic.
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.
Meteora herself is of course essential. She has a complex backstory and a lot of potential for character development. Star would also be a great addition as she and Meteora have an interesting dynamic in the show. Their interactions could be explored further in a fanfiction.
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.
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.
There could also be new villains created specifically for the fanfiction. For example, a dark magic user who wants to use the unborn child's magic for their own evil purposes. They could try to kidnap Star or find a way to transfer the baby's magic to themselves.
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.
There could be a fanfiction where Meteora has to face a new, more powerful threat in the 'Star and Forces of Evil' universe. This would put her in a position where she has to use her skills and intelligence to save the day. For example, a mysterious dark force from another dimension invades and she's the only one who can stop it. It would be full of action and adventure.
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.