Sonic, Sonia, and Manic are usually the main focus. They are the core of Sonic Underground and in the reunited fanfiction, their relationships and how they come back together are often explored.
Of course, Sonic is a major character. His growth since they last parted ways, his new skills, and how he impacts the reunion are often key elements. Sonia also plays an important role. Her reaction to seeing Sonic again and how she has changed in his absence can be a big part of the story. And Manic, with his unique personality and skills, adds to the dynamic of the reunion as well.
Well, perhaps in the fanfiction, the reunion is not as smooth as expected. There could be some tension among the characters due to past differences. Sonia might be angry at Sonic for leaving them years ago, and Manic could be caught in the middle. They then have to work through these issues while also facing a new threat that has emerged in their absence.
Well, obviously Bella is a main character. Often, there might be a tough trainer or mentor figure who helps her improve her fighting skills. For example, an old and experienced fighter who sees potential in Bella.
Panty and Stocking are the main ones. Writers like to take these two characters and put them in different scenarios. They might change their personalities a bit, but still keep their core identities. Fans are interested in seeing how they react in new situations, whether it's a different kind of battle or a strange encounter with a new character.
Naruto is usually a main focus. Since the concept is related to perfection, his journey to become the best Hokage and the strongest ninja is often explored in greater detail. He might be shown overcoming his flaws more quickly or mastering new techniques faster. Sasuke is also likely to be a central character. His path of revenge and his own pursuit of power can be re - imagined with the idea of perfection, like achieving the ultimate Sharingan or new levels of his combat skills.
Often, Bella is still a central character. Even in fanfictions about moving on, her journey is important as she might be dealing with the aftermath of her relationship with Edward or Jacob. Her growth and how she adapts to new situations are key.
Sometimes, even the family pets or the neighbors can be focused on in fanfictions related to this episode. They could be the ones who help Lincoln figure out what's really going on with his bad luck or they could be affected by his luck in some comical ways.
Ruby is often a main focus. Her leadership and combat skills can be explored in new ways in the 'Warm Bite' setting. She might be the one leading the charge against new threats.
Jacob can also be a main character in this type of fanfiction. His unique position as a werewolf and his relationship with Bella and the vampires make him an interesting character to explore in the context of changing tides. He might be trying to protect his tribe in a new situation or deal with his feelings for Bella in a different way.
Ruby is often a main focus. Her bright and positive nature can be explored more deeply in the context of these coloured dreams. Her relationship with her scythe and her journey as a leader can take on new twists in the fanfiction.
Most likely Bella and Edward would still be main focuses. Since they are the central couple in the original Twilight series, in the 'written in heaven' fanfiction, their relationship could be explored further, perhaps in a more spiritual or otherworldly way.