Gandalf would be a great character to focus on. He's a very powerful wizard in Middle - earth already. If he were to interact with the world of Harry Potter, he could offer a different perspective on magic. He might be able to teach the Hogwarts professors new ways of using magic. Also, his knowledge of the greater good and his ability to guide others could be really interesting when applied to the situations in Harry Potter, like guiding Harry in his fight against Voldemort in a more spiritual and less combat - oriented way.
I think it would be really interesting to focus on Hermione. She's so intelligent and curious. In Middle - earth, she could study the different magic and lore there. She might be the one to figure out how to combine the magic of the two worlds. For example, she could try to find a way to use the magic of Middle - earth to defeat Voldemort or help the Fellowship in a different way.
Definitely Harry Potter and Frodo Baggins. Harry has faced his own share of dark forces in the form of Voldemort, and Frodo has carried the burden of the One Ring. They could bond over their experiences of being thrust into dangerous and world - saving quests. Another interesting pairing would be Hermione Granger and Bilbo Baggins. Hermione's thirst for knowledge and Bilbo's love for stories and adventures could lead to some great exchanges.
I think Legolas could be really interesting. In 'The Hobbit', there are no elves from Mirkwood really involved in the story in the way Legolas is in 'The Lord of the Rings'. So, if you cross over the two, you could explore how Legolas might interact with the dwarves in a different context, considering the historical tension between elves and dwarves. He could also use his skills as an archer in new and unexpected ways, perhaps influenced by the events in 'The Hobbit' that he hears about from Gandalf or other characters.
Hermione Granger would also be an interesting character to focus on. She's extremely intelligent and curious. In a crossover, she would be the one trying to figure out how The Flash's powers work from a scientific and magical perspective. She might even try to find a way to combine magic and the Flash's speed for some greater good. Her inquisitive nature would make for some great interactions with The Flash and would add a lot of depth to the crossover story.
I think Harry Potter himself would be very interesting. Imagine him in Neverland, a place so different from Hogwarts. He could bring his courage and sense of justice from the wizarding world and apply it in the new setting. His magic skills would also be fascinating to see in the context of Neverland, like how he would use spells against the likes of Captain Hook or other threats there.
Definitely Harry Potter himself. His journey through time could be a way for him to gain more understanding of his parents and the events that led to their deaths. He could also try to change some things in the past to make the present better. For example, he could try to prevent Voldemort from ever becoming so powerful in the first place. And seeing Harry interact with the people from the past, like his grandparents or young Dumbledore, would be really fascinating.
I think Harry Potter himself would be very interesting. Imagining him learning tennis skills from the Prince of Tennis characters and using his wizarding abilities to add a new twist to the game. He could use his Patronus to guide the tennis ball or something like that.
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.
Leonidas from '300' would be a great character to focus on. Imagine his reaction to the magic of the Harry Potter world. He's a fearless leader, and seeing him interact with wizards and magical creatures would be fascinating. He might see the magic as a new weapon or a strange form of sorcery that he has to either embrace or fight against.
Luna Lovegood would be an unexpected but fascinating choice. Her unique perspective and belief in all kinds of magical creatures could make her very open - minded about the vampires and werewolves in the Twilight world. She might see them as just another type of magical being and try to find common ground, which could lead to some really interesting interactions and storylines.
I think Heero Yuy from Gundam Wing would be really interesting. His stoic and determined nature would contrast greatly with the more emotional and magically - driven characters in Harry Potter. He could bring a new sense of discipline and strategy to the fight against dark forces in the Harry Potter world.