Obviously, Harry Potter himself is a crucial character. He is the one 'chosen by death' and the whole story likely revolves around his experiences and growth. Hermione Granger would also be important. Her intelligence and research skills would be valuable in helping Harry understand his new situation. She might be the one to dig up ancient texts and lore about being chosen by death. Ron Weasley too, as Harry's best friend, would be there to support Harry, even if he doesn't fully understand what's going on at first.
In some versions, the plot could revolve around Harry exploring different magical realms that are related to death. He might enter a realm where the line between life and death is blurred, and he has to navigate through challenges there. This could involve facing his own fears and insecurities in a very tangible way. For instance, he might see visions of his parents and what could have been if they had lived, but he has to resist the temptation to stay in that false reality and return to the real world to fulfill his destiny.
There could be new original characters created specifically for this fanfiction. They might be mentors or allies to Harry in his journey. For example, a powerful and mysterious wizard who helps Harry understand the true nature of his being chosen.
Neville is a likely candidate. He has always been brave and has a strong connection to the Gryffindor values. If not for Harry, Neville could have easily stepped into the role of the main character. His growth throughout the series shows that he has the potential to be a great hero.
One idea could be that it explores the power dynamics within the wizarding world without Harry as the chosen one. Maybe other characters step up and new alliances are formed.
Well, Harry Potter is obviously one of the main characters. Since it's a fanfiction related to him being the 'Chosen of Avalon', he would be central to the story.
One interesting plotline could be Harry having unique magical abilities that are even more powerful than in the original books. For example, he might be able to control time in ways not seen before.
Perhaps in this fanfiction, Draco Malfoy has a change of heart and is revealed to be the one who can truly end the dark reign. He might be in a unique position to infiltrate the Death Eaters due to his family's connections, and then use that knowledge to turn the tables on Voldemort. His Slytherin cunning could be used for good in this alternative scenario.