A rogue magician who has a vendetta against Percy's family. He sees Percy's niece as an easy target to get back at Percy. He uses his magic to manipulate the world around her, causing confusion and danger. He might also be trying to steal her powers for himself to gain more influence in the world of magic and mythology.
Perhaps Percy's niece is suddenly transported to a different realm in the world of Greek mythology. This realm is in chaos due to some dark magic. She has to use her wits and any latent abilities she has to survive and find a way back. Meanwhile, Percy is on a quest of his own but starts to notice strange things that lead him to suspect his niece is in trouble. Eventually, their paths cross and they team up to save the day in this strange new realm.
Another character that could be included is Clarisse. Her tough - as - nails attitude could contrast with the niece's, and they could start off as rivals but eventually become friends. Also, Tyson, Percy's cyclops half - brother. He could be like a big, protective figure for the niece, and his unique abilities could come in handy during adventures or when the niece is in danger.
One idea could be that Percy's niece discovers she has a unique power that no one else in the family has. Maybe she can communicate with sea creatures in a different way than Percy. Another could be that she gets involved in a quest that was thought to be only for the main heroes in the Percy Jackson world, and she has to prove herself to the other demigods.
Perhaps a rogue demigod who has made a deal with a speed - based meta - human villain from the Flash's world. Together, they plan to take over both the magical realm and Central City. The demigod could use his powers in combination with the meta - human's technology to create some really tough challenges for Percy and the Flash.
If it's a crossover with Alice in Wonderland, the Queen of Hearts could be the main antagonist. The Queen might see Tinkerbell as an intruder and try to behead her. Tinkerbell would have to use her wits and magic to survive in the crazy world ruled by the Queen of Hearts.
It could be the warden of the prison. The warden might be a cruel and power - hungry person who wants to keep Percy imprisoned for his own reasons, like getting some sort of reward or power from those who put Percy there.
A powerful dark sorcerer could be the main antagonist. He might be seeking to use Percy's skills for his own evil purposes, and when Percy refuses, he becomes the target. The sorcerer would have a vast array of dark magic at his disposal, making him a formidable foe.
It could be a powerful mage who wants to use Percy's demigod powers for his own evil plans. This mage might be jealous of Percy's unique abilities and try to control him.
It could be a character who is jealous of the power that could come from combining the two worlds. Maybe a rogue demigod who wants to use Jessie's knowledge of her world's technology and the magic of the Percy Jackson world to rule over both. This character might start causing problems by stealing important artifacts from both worlds to try and build a device that would give them ultimate power.
A rogue group within Corneria's military could be the main antagonist. They oppose General Pepper's leadership and think that using his niece as a pawn will help them gain power. They might try to frame her for a crime she didn't commit and make the Star Fox team turn against her, while they secretly work on their own power - grabbing plans in the background.