They would probably interact the most with Harry, Ron and Hermione. Since these three are at the center of most of the action in the Harry Potter world, it's natural that if they were in the fanfic, they would be drawn to them. Harry is the hero, and they might want to help him in his quest. Ron is always up for an adventure and Hermione has the knowledge, so they could all form a great team together.
In a Harry Potter fanfic, if they had lived, they might interact a great deal with the teachers at Hogwarts. Professors like Dumbledore, McGonagall and Snape are important figures. Dumbledore could offer them wisdom and guidance, McGonagall might be strict with them but also teach them a lot, and Snape, well, they might have an interesting relationship with him, trying to figure out his true nature while also learning from him in Potions class.
I think they would interact a lot with the members of the Order of the Phoenix. People like Sirius Black, Lupin and Tonks. These are all characters who are actively fighting against Voldemort. If they had lived in the fanfic, they would likely join the fight, and thus interact closely with these members. They could learn from Sirius about his family's secrets, from Lupin about being a werewolf and from Tonks about her unique magical abilities.
Well, if they had lived in Harry Potter fanfic, the story could have taken a much more complex turn. Maybe they would have joined Harry in his fight against Voldemort. Their unique skills or knowledge could have given the Order of the Phoenix an edge. For example, if they were really good at potions, they could have brewed some powerful antidotes to the dark magic-induced poisons that Voldemort's followers used. It might also change the relationships among the characters. Harry might have found new friends or allies, which could have affected his decisions and actions throughout the series.
Harry Potter himself could interact with Sophie from Huntik. Harry is used to facing great challenges and having a strong sense of justice. Sophie, with her connection to the Titans, could find common ground with Harry in their fight against evil. They could learn from each other's worlds and work together to overcome a powerful dark force that threatens both universes.
Obviously, Harry Potter and Clark Kent would interact a lot. They are both iconic characters in their own universes. Clark, with his superpowers, would be curious about Harry's magic, and Harry would be interested in Clark's abilities. Also, Hermione Granger, if she somehow also ends up in Smallville, would likely interact with Chloe Sullivan. They are both intelligent and inquisitive, so they could bond over solving mysteries or studying the differences between their two worlds.
Rebekah Mikaelson and Hermione would be likely to interact. Hermione is curious and intelligent, and she would be intrigued by Rebekah's different kind of magic. She would probably ask Rebekah a million questions about her vampire nature and the magic in her world. Rebekah, on the other hand, might be a bit annoyed at first but would come to respect Hermione's thirst for knowledge.
Probably Harry Potter and Indiana Jones themselves. Harry's knowledge of magic and his experience in fighting dark forces could complement Indiana Jones' skills in exploration and combat outside of the magical realm.
Hagrid, of course. He loves all kinds of strange and dangerous creatures. He would be the first to be intrigued by the Indominus Rex and would probably try to approach it to see if he could take care of it or at least understand it better.
Obviously, Harry Potter and Riddick would interact. Harry, being the main hero in his world, would be curious about Riddick's strange abilities and his rough - and - tumble way of life. Riddick, on the other hand, would likely be suspicious of Harry at first but then come to respect his magical powers.
I think Harry and Abby would be really interesting. Abby is so into science and forensics, and Harry could introduce her to the world of magic. She might be the most open - minded among the NCIS agents to Harry's abilities. They could combine their skills, like Abby using her knowledge of technology to help Harry understand Muggle - related aspects of the case while Harry uses magic to uncover things that Abby's science can't explain.
Definitely Harry Potter and Finn Hudson. Harry is the hero in the wizarding world, and Finn is a key figure in the Glee club. They could bond over their leadership roles. Harry could teach Finn some magic tricks, and Finn could show Harry how to be more confident while performing in front of a large audience.
Hermione Granger could have an interesting interaction with Batman. Both are highly intelligent and strategic. Hermione could share her knowledge of magic and ancient texts, while Batman could teach her some detective skills and how to use technology in combat situations.