Horus himself would be a central character. He would bring his own set of powers and a whole different mythology into the story. Dumbledore could also play a significant role. He's wise and powerful, and he might have some knowledge about Horus or the connection between the two worlds. Maybe he's been waiting for Horus to arrive for some greater purpose. And then there could be some minor characters like Neville Longbottom who could end up being unexpectedly helpful in the journey.
In addition to the regular Harry Potter characters, we could have some Egyptian deities or spirits related to Horus appear in the fanfic. They could be trying to reach out to Horus or be part of the reason he can't return home. And from the Harry Potter side, characters like Sirius Black (if he's still alive in the fanfic's timeline) could be involved. He has his own brand of magic and could be a great asset in protecting Horus and helping him on his journey home.
The plot might revolve around a dark force in the Harry Potter world that has something to do with Horus's inability to return home. For example, a dark wizard could have trapped Horus for some evil purpose. Harry then discovers this and has to battle the dark wizard while also finding a way to free Horus and send him home. There could be epic duels and magical mysteries to solve along the way.
Obviously, the four founders - Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin would be main characters as they are the ones returning.
Hermione Granger and Bella Swan could also be main characters. Hermione could bring her intelligence and knowledge of magic, while Bella could provide insights into the Twilight world. They might start off as rivals, with Hermione thinking Bella is too naive and Bella finding Hermione too bossy, but then they grow to respect each other and work together.
Harry Potter himself could be the main character. He could be the one who stumbles into the world of Bioshock. His magical abilities would be put to the test against the threats in Rapture.
Definitely Harry. He could form a special bond with a particular Pokegirl, maybe a fire - type Pokegirl, and they could go on adventures together in the wizarding world. Ron could also be a main character. He might be a bit more hesitant about the Pokegirls at first but then grow to like them. And there could be a new character, a Pokegirl trainer who comes to Hogwarts to introduce the concept of Pokegirls to the wizards, and she becomes an important part of the story.
Chef Ramsay would be the central figure. Then perhaps Hagrid could be a character. Hagrid is always dealing with magical creatures and he could provide some very interesting and unique ingredients for the kitchen. Also, Neville Longbottom could be there. He has a love for plants, and he could contribute by providing fresh magical herbs for cooking. And of course, the Hogwarts head chef, who would be resistant to change at first but gradually learn from Ramsay.
There could also be new characters introduced in the fanfic. Maybe a young, talented wizard or witch who is just starting at Hogwarts and becomes involved in Harry's return journey. They could bring a fresh perspective and new abilities to the story. And of course, Dumbledore, even if he's not physically present all the time, his influence and legacy are likely to play a role, so he can be considered one of the important characters in a sense.
A powerful dementor - like being that has been corrupted by Asgardian dark magic. This entity could be draining the life force of both wizards and Asgardians. It would be up to Thor and Harry Potter to find a way to stop it, as it poses a threat to both of their worlds.
Obviously, Harry Potter and Merida would be the main characters. Harry with his knowledge of the wizarding world and his courage, and Merida with her strong - willed and adventurous spirit. They could team up to face challenges that require both magical and non - magical skills.
Obviously, Harry Potter could be one of the main characters. He could be the one to bridge the gap between the two worlds, using his courage and magic skills. Master Chief from Halo is also a great candidate. His combat experience and leadership would be valuable in the wizarding world full of new threats. Hermione would also be important as her intelligence could help in understanding the Halo technology.