Edward and Bella are likely to be main characters. Since they are central to the 'Twilight' story, in a fanfiction about a living will, they could be involved in making decisions regarding the living will, perhaps related to their future together or the safety of their family.
Another possibility is that the fanfiction could introduce new characters who play a major role in the 'coming home' story. They could be new vampires or humans who have a connection to the original Twilight characters. For instance, a new vampire who was turned while Bella was away and now she has to deal with this new presence when she comes home, and this new character could bring a whole new set of conflicts and relationships into the story.
There could be smugglers like Han Solo was in the original Star Wars. They often operate in the shadows, doing deals that are not exactly legal. Another likely character type could be bounty hunters. They work in the fringes of society, chasing after targets for money.
It's likely that Bella and Edward could be main characters. Since it's a Twilight fanfiction, they are central to the original story. And with the theme of 'broken hearts', their tumultuous relationship could be a focus. Maybe they are on a journey along the highways, either physically or in their relationship, and facing heartbreak.
Well, Bella and Edward would probably still be main characters. Since it's Twilight - related, their relationship is central. They might be exploring new aspects of their relationship in this 'whole new life' scenario.
Bella Swan is a very likely character. Since she has such a strong connection to Forks, which is her home in the Twilight world. Her journey home could be filled with thoughts of Edward and the vampires she has come to know.
Maybe an old storyteller who has a deep connection to the area with the cracked pavement and during twilight shares tales of the past with the younger generation. There might also be a group of friends who use the cracked pavement as their meeting place at twilight, and they get into all sorts of adventures together.
The main characters could also include some of the more powerful or unique characters from each series. In Harry Potter, Dumbledore with his great wisdom and magical power might play a key role. He could be the one trying to figure out how to deal with the Twilight - related elements in this crossover. In Twilight, the Volturi, with their control over the vampire world, could be main characters. Their actions and decisions would likely have a big impact on the story as they interact with the wizards. Additionally, characters like Jacob from Twilight, with his werewolf nature, would add an interesting element when interacting with the werewolves in Harry Potter (if any are included in the fanfic).
The baby itself could be a sort of main character in a way. It might have its own unique personality, even as a baby. And it could be the focus of the story as the characters try to protect it, raise it, and figure out what kind of future it will have in the Twilight world.
The main characters would most likely still be the Cullens. Edward, Bella, and their daughter Renesmee. Also, Jacob would probably be a major character as his relationship with the Cullens and Renesmee was a big part of the story.
Jacob Black could also be a main character. His relationship with Bella and the werewolf pack's connection to the Cullens might be explored in a new light in this fanfiction. Maybe his transformation or his knowledge of the supernatural world comes into play earlier than in the original story.