Annabeth from Percy Jackson could be really interesting. She's smart and strategic. In a crossover, she could be the one to figure out how the H2O girls' powers work and how they can be used to everyone's advantage. Also, Cleo from H2O has her own charm. She comes from a different background and her insecurities combined with Annabeth's confidence would make for great character development.
I think the H2O mermaid, Rikki, could be a great focus. She's feisty and independent. If she were to meet Harry Potter, she'd probably be both intrigued and a bit skeptical of his magic. Her interactions with the wizarding world characters would be full of drama and discovery as she tries to understand this new and strange magic.
Definitely Percy Jackson himself. His ability to control water and his overall demigod powers would make for some really cool interactions in the Halo universe. He could be a bit of an outlier among the Spartans, but his courage and quick thinking would be assets. Also, Master Chief. His stoic and badass nature would contrast well with Percy's more impulsive and humorous side. When they team up, it would be an amazing duo to write about.
Annabeth Chase could be an interesting character to focus on. She is highly intelligent and strategic. In a crossover, she could use her knowledge of architecture and battle strategies to help Eragon and his allies. She might study the magic in Alagaesia and find similarities or differences with the powers of the demigods. Her relationship with Percy could also be explored in a new light, especially when they are in a completely different world. How would they support each other in this new and strange environment? And how would she interact with the characters in Eragon's world?
Well, perhaps the H2O girls are being chased by some sea monsters that are normally under Percy's jurisdiction. Percy and his demigod friends come to their rescue. In the process, the H2O girls learn about the world of Greek mythology and demigods. They could then join forces to fight off other threats that come their way, like an evil sea god who wants to take over both their worlds.
Annabeth Chase would be an exciting character to center on. Her intelligence and strategic thinking would be a great asset in the maze. She could quickly analyze the layout of the maze and come up with plans. She might also be the one to bridge the gap between the two groups, as she's good at understanding different people and situations. Her knowledge of architecture and mazes from her own world could be applied to solving the mysteries of the Maze Runner's maze. And her relationship with Percy would add an extra layer of drama, especially when dealing with the new and dangerous situations in the crossover world.
Edward and Cleo would also be a fascinating pair. Edward's brooding and mysterious nature combined with Cleo's kind - hearted and sometimes naive mermaid self could lead to some really interesting interactions. Maybe Edward could be intrigued by Cleo's water - based powers and Cleo could be both scared and attracted to Edward's vampire side.
I think pairing Hermione with one of the H2O girls would be really interesting. Hermione is so studious and knowledgeable about magic, while the H2O girl has her own unique water - related powers. They could learn a lot from each other. For example, Hermione could try to figure out the scientific basis behind the H2O girl's powers, while the H2O girl could show Hermione some new ways to interact with water magically.
Nico di Angelo would be an interesting choice. His dark and brooding nature along with his powers over the dead would be a contrast to the more light - hearted aspects of the Harry Potter world in some ways. He could introduce the wizards to the concept of the Underworld and the spirits. And his ability to shadow - travel could be used in some really creative ways, like getting the group in and out of dangerous situations quickly, whether it's in Hogwarts or in the magical areas related to Greek mythology.
I think Mal from Descendants would be really interesting. If you combine her with the Percy Jackson universe, her skills in magic and her somewhat villainous past could create a lot of drama. She could be a bit of an outcast among the demigods at first because of her heritage, but then prove herself as a valuable ally. For example, she could use her magic to help the demigods in a battle against a new type of monster that is resistant to normal demigod powers.
I think Percy Jackson himself would be really interesting. He has such a strong personality and his powers are unique. In a fanfiction with Descendants, he could bring his demigod skills and his sense of adventure. He might be a bit of a fish out of water in the Descendants' world, which could lead to some really funny and exciting situations. His loyalty and determination would also make him a great character to drive the story forward.