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.
Harry Potter himself could also be likely to interact with the Indominus Rex. Given his heroic nature, when the creature poses a threat to Hogwarts or the wizarding world, he would step up. He would use his skills and the help of his friends to try and figure out a way to deal with the situation involving the Indominus Rex.
Naruto might try to befriend the Indominus Rex. He's always been one to see the good in others, even enemies. So he'd probably approach it with an open heart. Sasuke, on the other hand, would be more cautious and likely see it as a threat to be eliminated if necessary.
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.
Hermione Granger. She's very curious and intelligent. When she sees Ghost Rider, she would be eager to learn about his powers and where he comes from. She might try to research in the Hogwarts library if there's any mention of his kind of being, and then try to communicate with him using her vast knowledge.
Harry Potter himself would be likely to interact with Elsa. Given his own experiences with unique powers and his role as a hero, he would be sympathetic to Elsa's situation. They could bond over their shared sense of responsibility and their ability to do things that others can't. Harry could also introduce Elsa to his friends and the world of Hogwarts.
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.
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.
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.
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.
One possible scenario in such a fanfiction could be that Harry discovers his unique Animagus form of Indominus Rex during his time at Hogwarts. Maybe it's a result of some dark magic experiment gone wrong in the Forbidden Forest that alters his magical core and gives him this fearsome form. He would have to hide it from his friends and teachers at first, fearing the consequences.