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.
If we swap Star and Marco, then the main antagonists could be those who were previously minor annoyances for the other character. For example, if Marco is now in Star's role, Ludo, who was a main antagonist for Star, could be an even bigger threat as Marco might not have Star's natural magical intuition to deal with him.
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.
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.
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.
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, obviously Star is one of the main characters. She is often pitted against Toffee in the show, and in fanfictions about Toffee's abs, she would be trying to figure out his secrets which might be related to his physical features like his abs.
Star is the key protagonist as she has the power and the will to take on any challenges related to the Laser Puppy. The Laser Puppy is not just a pet - like character but also has its own charm and mystery that makes it a main character. Other characters like Pony Head, Star's best friend, might also be involved. She could add some comical relief while also being helpful in some of the adventures that revolve around the Laser Puppy.