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.
Well, perhaps she's on a quest. The Avengers mistake her for a threat at first. She's looking for a magical artifact that has the potential to either save or destroy both the demigod and superhero worlds. Along the way, she teams up with some of the younger Avengers like Spider - Man or Shuri. They don't trust each other fully at the start, but as they face various challenges together, they start to respect and rely on one another. And she's constantly in touch with her demigod family, getting advice from them about how to handle the superhero situation.
She might bring in her demigod skills. For instance, her knowledge of ancient languages could help the Avengers decipher some mysterious inscriptions they find. Also, her agility and combat training from the demigod side could make her a formidable fighter in the Avengers' battles.
One way could be to have Percy Jackson's powers of being a demigod interact with the superpowers of the Avengers. For example, his ability to control water could be used in a joint mission with Thor's lightning powers. Maybe they are facing a common enemy that can only be defeated by combining their unique skills.
In such a crossover, there could be epic battles. Percy might use his water - controlling powers against the Avengers' enemies. Maybe Thor and Percy would have an interesting encounter, comparing their powers over elements like water and lightning.
Character development is another great aspect. In these crossovers, characters from both universes often have to step out of their comfort zones. Percy might have to deal with the more cut - throat and modern world of the Avengers, while the Avengers have to understand and accept the existence of magic and gods. This forces them to grow and change in new ways, which is always exciting to read about.
You can start by giving them unique personalities. For example, one could be really shy but have a hidden strength. Then, put them in different situations to show how they grow. Like when they face a monster, the shy one might find the courage to fight.