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.
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.
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.
Another possibility is that Harry interacts with some of the administrative staff in the DMLE. They might be the ones who first take his call and record the details. They could play a role in passing the information on to the relevant people within the department, and Harry might have to convince them of the urgency of the situation.
The characters would interact in very strange ways. The Hogwarts students would be shocked by the Jackass crew's behavior. The Gryffindors might be initially intrigued, while the Slytherins would be disdainful. Harry and his friends would probably try to keep the Jackass guys out of trouble, but it would be difficult as they are always looking for the next wild thing to do.
Harry might be in awe of Elsa's powers at first. He would likely approach her with caution and respect, as he does with most powerful beings. Elsa, on the other hand, would be curious about Harry's world of wands and spells. They could start by sharing their stories. Harry would talk about Voldemort and his battles, and Elsa would share her experiences with her frozen kingdom and her isolation.
The Stargate characters are used to dealing with advanced technology and alien cultures. When they meet the Harry Potter characters, they might be initially confused by magic. For example, Jack O'Neill might be both amused and intrigued by the spells the wizards cast. Meanwhile, Harry Potter and his friends would be curious about the Stargate technology and the different worlds it can access.
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.
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.