Jacob and Emma could be paired up. Jacob's hot - headedness and his werewolf abilities would contrast with Emma's more calm and collected mermaid nature. They could start off as enemies but then gradually develop a mutual respect as they face challenges together 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.
I think pairing Hermione with Edward would be really interesting. Hermione's intelligence and love for knowledge could be contrasted with Edward's brooding and his vampire nature. She could be intrigued by his abilities and he by her magic.
Edward Cullen and The Flash would make for an exciting pair. Edward has his vampire powers, including super - strength and enhanced senses. When combined with The Flash's super - speed, they could be an unbeatable team. Imagine them racing against each other at first, testing each other's limits, and then realizing they can achieve more together. Also, Edward's brooding nature and The Flash's more upbeat and positive attitude would create a great contrast in the story.
Harry Potter and Edward Cullen would be an interesting pair. Harry has his own inner struggles with the power of the Dark Lord inside him, and Edward has his battle with his vampire nature. They could learn from each other about self - control and using their powers for good. Also, they both have a strong sense of protecting their loved ones.
Ron and Jacob could be a fun pair. Ron is a bit of a food - loving and sometimes clumsy character, while Jacob is a hot - headed werewolf. They could start off bickering but then end up becoming friends. Ron could introduce Jacob to some of the delicious wizarding treats, and Jacob could show Ron how to embrace his more wild side in a different way than magic allows.
Another interesting pairing could be Alice Cullen and Sookie Stackhouse. Alice has her visions of the future, and Sookie has her mind - reading ability. They could team up to solve some mystery that involves both the 'Twilight' and 'True Blood' worlds. Maybe they could use their powers to prevent a war between the two different vampire factions.
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.
Emma from H2O and Mimmi from Mako Mermaids would be an exciting pair. Emma is the smart and organized one in the H2O group. Mimmi is also intelligent but has a more free - spirited nature. They could work together to figure out some mystery in the ocean world. Maybe they find an ancient mermaid artifact and have to use their combined knowledge and skills to unlock its secrets.
Sonic and Pikachu would be a really interesting pair. Sonic's high - speed and adventurous nature combined with Pikachu's electric powers and cute but brave personality could lead to some really fun and action - packed scenes. They could race each other and then work together to face challenges.
Naruto and Tsukune would be an interesting pair. Naruto's energetic and determined nature combined with Tsukune's kind heart and his own growth as a character in the Rosario world could lead to some great interactions. They could learn from each other's ways of dealing with challenges and protecting their friends.