The main characters are clearly young Joseph and Dio. Joseph is a character with a strong will and unique abilities, especially his Hamon. Dio, on the other hand, is a powerful vampire with his own set of formidable powers.
Perhaps a plot point where the power dynamic between them shifts unexpectedly. Maybe Joseph discovers a weakness of Dio that no one else knew, leading to a very tense and exciting confrontation. In fanfiction, this could be something completely new and creative that the author invents.
Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.
No. Ryomen Sukuna is a fictional character from Japanese mythology and modern manga/anime. He doesn't have a basis in a true historical event or person.
Nope. Ryomen Sukuna has no basis in reality. It's a product of the creative minds of the writers and illustrators who designed a captivating fictional character for the enjoyment of the audience.