Harry himself is very likely to be involved. Given his many encounters with death, from facing Voldemort to losing loved ones, he has a deep connection to the concept. He would be exploring the aspect of death perhaps to gain more power to protect his friends and the wizarding world.
Dumbledore, even though he has passed away, could still be a major part of the fanfiction. His wisdom and past experiences with death, like his sister's death, could be explored further. His spirit or memories could guide the living characters in their exploration of the aspect of death, providing cryptic clues and moral lessons along the way.
Obviously, Harry Potter himself would be a main character since the title revolves around him. Hermione and Ron are also very likely to be involved. They are Harry's closest friends throughout the series and would surely play a part in a story where Harry is welcoming death. They might try to stop him or support him depending on the direction of the fanfiction.
Voldemort would also be a key character. His obsession with immortality and his actions that led to many deaths in the series are fundamental to the theme of life and death. In a crossover, he could be pitted against new threats or forces that have different views on death, which would make for an interesting story.
Definitely the Weasley twins, Fred and George. They are known for their love of mischief and innovation. They would be all over the idea of street racing in the wizarding world. They could create all sorts of magical gadgets to use during the races. Another character could be Draco Malfoy. He would likely be a rival racer, using his family's wealth and influence to get the best magical racing equipment.
Definitely Harry Potter himself. He's the central character in his own series, so it's likely he'd be involved in any crossover. Hermione Granger could also be a key character as she's smart and curious, and would be interested in learning about the 'Twilight' world. From 'Twilight', Edward Cullen is an obvious choice. His brooding nature and unique abilities would make for interesting interactions with the 'Harry Potter' characters. Also, Jacob Black could be involved, especially when it comes to the 'imprint' aspect as he has his own story arc related to that in 'Twilight'.
Obviously, Harry, Hermione and Ron are the most likely. They are the main characters and are always together, so it's natural for them to be playing truth or dare. Neville could also be involved as he's their friend. And Ginny might join in too, especially as she gets older and becomes closer to the group.
Hermione Granger is also a likely character. Her intelligence and love for knowledge could lead her to discover a way to bridge the past and the future. She might use her Time - Turner knowledge or find some new magical method. And she could play a key role in helping past characters adapt to the future or vice versa.
Obviously, Harry Potter himself is the central character as the one being abused and neglected. The Dursleys are also key characters as they are the ones who abuse and neglect him in the original story. Hermione and Ron might also be involved as they could be the ones to discover Harry's situation and try to help him. Dumbledore could also be a part of it, perhaps as someone who failed to notice or prevent Harry's abuse initially.
Obviously, Harry Potter himself. Hermione would probably be by his side, helping him research about his claim to lordship, using her vast knowledge from the library. Ron might also be there, providing moral support and his unique sense of humor in difficult situations.
Neville Longbottom could be a key character. He might take on a bigger role in Hogwarts in Harry's absence and also be in touch with Harry, providing support from the school. Ginny Weasley would also be involved as she has a close relationship with Harry. Additionally, some of the Hogwarts professors like Professor McGonagall could play important parts, either trying to bring Harry back or helping him in his mission outside the school.
Obviously, Harry Potter himself is the central figure. His friends like Ron and Hermione would also be involved. They would support Harry in his revenge plans, with Ron providing his loyalty and Hermione using her intelligence to strategize. Ginny Weasley might also be part of it, as she is Harry's love interest and a strong - willed witch in her own right.