In terms of the Death Eaters, if they were to enter the RWBY universe, they could pose as a new type of threat. They might be attracted to the power of Dust and try to corrupt it for their own dark purposes. This would put the RWBY characters and the Harry Potter characters on the same side, fighting against the Death Eaters' evil plans. The Harry Potter characters would have to adapt their combat skills to deal with the Grimm as well as the Death Eaters, and the RWBY characters would have to learn about the dark magic of the Death Eaters and how to counter it.
Harry Potter himself could be seen as a kind of foreign wizard in the RWBY world. He might be initially confused by the concept of Dust but quickly adapt and use his magic to enhance the RWBY team's capabilities. Hermione would probably be fascinated by the study of Dust and Semblances, and use her vast knowledge to help bridge the gap between the two magic systems. Ron could be the one who forms quick friendships with the RWBY characters, sharing his sense of humor and loyalty.
The Hogwarts professors could also play interesting roles. Dumbledore, for instance, might be seen as a wise and mysterious figure in the RWBY universe. He could offer guidance to Ozpin on matters of leadership and the handling of powerful magic. Snape could be a more complex addition. Maybe he's initially seen as an antagonist in the RWBY world due to his cold demeanor, but later proves to be an ally, using his potions knowledge to create new substances related to Dust. And the students from Hogwarts, like the Weasley twins, could bring their prank - making skills to the RWBY academy, adding some light - heartedness to the story.
The characters from Hotel Transylvania could be new students at Hogwarts. Dracula could be a teacher of some sort, perhaps teaching about dark creatures in a different way than Hagrid. His bat form could be mistaken for a dark omen by some students at first.
It would be a very difficult fit. Since the wizarding world has its own norms like monogamous relationships in most cases (like the marriages we see in the series). If one were to introduce concubines, it would have to be in an alternate universe where the social and moral values are completely different. Maybe in a fanfic where the wizarding world is modeled after a more ancient and permissive society, but this would be a huge departure from the original feel of the Potterverse.
Mileena could be a foreign exchange student of sorts from a different magical realm. Her unique appearance and combat skills would make her stand out at Hogwarts. She might have trouble adjusting to the more academic and rule - based magic of the wizarding world compared to her own more instinctive and combat - focused magic.
He could fit in as a unique hero. His magic could be seen as a special power like the others in DC. For instance, he could use his Patronus charm to fight off fear - inducing villains.
Alucard could be a part of an ancient prophecy in the Harry Potter fanfiction. His arrival in the wizarding world could be foretold. He might be seen as a savior or a bringer of chaos. His powers could be seen as a necessary evil to defeat a greater threat, like a new and more powerful form of Voldemort. He would bring his own brand of violence and mystery to the relatively more structured world of Harry Potter, making for a very exciting and unpredictable story.
Hawkgirl could be a new kind of magical creature. Her ability to fly with her wings would be a unique form of magic in the wizarding world. She might be a protector of some hidden magical place in Hogwarts or the Forbidden Forest, like a guardian of an ancient magic that Voldemort is after.
In a Pirates of the Caribbean fanfiction, Ruby could be seen as a sort of chosen one. Her innocence and determination could be what's needed to break a powerful curse. Weiss might form an unlikely alliance with a crusty old pirate who teaches her the ropes of the sea. Blake could discover a hidden community of Faunus - like beings in the Pirate world, and Yang would befriend some of the rowdiest pirates and show them that there's more to strength than just brute force.
Perhaps she could be a part of a hidden underworld within the wizarding world that Harry and his friends are unaware of. The female demon could be a key to unlocking secrets about this underworld, like ancient spells or hidden treasures. Her presence could stir up trouble among the wizarding community who are either scared of her or want to use her for their own gain.
She could fit in as a mysterious visitor. For example, she might show up during a Quidditch match and use her magic to save a player from a dangerous fall. Her presence would be a shock to everyone as her magic seems so different from what they know. She could then start to interact with the characters, sharing her own brand of magic with them, like teaching Hermione how to make a room clean itself with a simple song.
The Cyber Knight could be part of an ancient prophecy in the Harry Potter fanfiction. The prophecy could state that a being of metal and magic will come to save or destroy the wizarding world. His arrival then sets off a chain of events. Some would see him as the savior, while others, like the Death Eaters, would see him as a threat to their pure - blood ideology. He would have to prove himself worthy of being on the side of good, all while trying to figure out his own place in this strange new world of magic.