Perhaps it's a person on the verge of death who has some sort of premonition or awareness that death is not the end. This character could be fighting against their impending death in the physical world while also being drawn towards this new 'beginning' that awaits them, creating an internal conflict within the story.
It could be a reaper - like a Grim Reaper type of character. Since the story is about death being a beginning, a reaper could be guiding the newly dead into this new phase, and the story could be told from their perspective as they deal with different souls and the concept of this new start.
It could be an old man who has lived a full life and is not afraid of death. When he dies, he becomes the main character in this new 'beginning' and we follow his adventures in the afterlife.
One possible theme is the exploration of the unknown. Since death is typically the great unknown, and this title says it's the beginning, the novel could be delving into what lies beyond death. It might take readers on a journey into uncharted territories, be it spiritual, psychological, or some other realm that we don't understand in our normal perception of life.
It could be a young adult who has just graduated from college and is starting their career. This is a new beginning for them as they enter the professional world, face new responsibilities, and strive to achieve their dreams.
I'm not sure as I haven't read the novel specifically, but typically in a novel with such a title, there might be a character who has recently died or is on the verge of death and is about to embark on this 'new beginning'. There could also be some sort of guide or mentor figure to help them through this transition.
The main character could also be a new character introduced into the fanfic's universe. For instance, in a Lord of the Rings - style fanfic, a young elf who has grown up in a secluded part of the forest. One day, they decide to leave their home and start a new path, which leads them to get involved in the larger events of the war against Sauron, perhaps by forming an alliance with humans and dwarves.
It could be a young detective who is trying to solve the mystery of the Death Note's reappearance. He might be really smart and determined to stop whoever is using the Death Note for evil purposes.
The main character might be an ordinary person who has made some bad choices in life. For instance, they got involved with the wrong crowd and are now in danger. To protect themselves and their loved ones, they choose to fake their death. In this 'I chose to fake my death novel', it could show how an ordinary person copes with such an extreme situation.
Harry Potter himself could be the main character. Since he has faced death so many times in the original series, it would be interesting to see how he interacts with the concept of Death from the Endless.
It could be a Slytherin student who has always been drawn to the darker aspects of magic. They might have grown up in a family with a history of dark magic, and this has influenced their view of death as something to be embraced rather than feared.
It could be the underdog character from the original comic who finally gets a chance to shine in this fanfiction. For example, if it's a superhero comic, it might be the sidekick who has to step up when the main hero is incapacitated and is faced with a life - or - death situation.