There may be new characters introduced as well. Maybe a mysterious figure who has a special ability to either enhance or dampen Naruto's will to live. This character could add an extra layer of mystery and conflict to the fanfiction.
Sasuke. He has such a complex character arc and his death would have a huge impact on the story. So, in fanfiction, he's a likely candidate to come back to life and continue his journey. Maybe he has some unfinished business or a new purpose to fulfill.
Obviously, Naruto would be one of the main characters. Since it's a Naruto - centered fanfiction with the beyonder element added. Sasuke could also be a main character as he is often Naruto's counterpart and rival. They might both be involved in exploring the beyonder powers. Additionally, characters like Kakashi could play important roles. He could be the one guiding Naruto and Sasuke in understanding these new and strange beyonder powers, given his experience and wisdom.
Naruto would be the central figure. But in this British version, there could be new characters added, like a British wizard - type character who becomes Naruto's mentor in understanding the new 'British - ninja' aspects of his life. And of course, Sasuke and Sakura would be adapted to fit the British setting, perhaps with different family backgrounds that are more in line with British culture.
Definitely Naruto himself. He'd be a central character, causing chaos and bringing energy to the daycare. And Sasuke would also be there, probably being a bit of an aloof kid that Naruto is determined to get close to.
I think Kakashi would be an important character too. He could be Naruto's mentor in this new situation, teaching him how to use his remaining abilities effectively. Also, Sakura might be there, using her medical ninjutsu skills to try and help Naruto with his crippled state.
The main characters would surely include Naruto himself. His child or children are bound to be in the spotlight. In addition, any of the Akatsuki members who may have reformed or are still causing trouble could be part of the story. For example, if Naruto's child has some connection to the Akatsuki's past actions or secrets, it could bring them into the plot. Also, the villagers of Konoha, who would interact with Naruto's family, could be important characters in this fanfiction.
Besides Naruto and Sasuke, Sakura could also be a main character in such fanfiction. Her medical ninjutsu skills might be needed to heal the wounded during the battles against Galactus. And there could be some original characters created specifically for the fanfiction who have some special knowledge or power related to Galactus and the Naruto world.
Other Naruto characters like Sasuke or Sakura might also be important. Sasuke could be involved in Naruto's new role as a 'guardian' either as a supporter or a skeptic. Sakura, with her medical skills, could play a role in helping Naruto in his 'guardian' - related adventures, perhaps by healing those affected by new threats that come with the combination of the two universes.
Obviously Naruto and Ophis. Naruto is the main protagonist from the Naruto series, and Ophis is from another fictional context. There could also be some of Naruto's friends like Sasuke or Sakura who might be involved in some way. Maybe they are worried about Naruto when he disappears into Ophis' world.
There could be new characters introduced in this fanfiction. Maybe a mysterious ninja from a far - off land who knows about Naruto's secret and either helps or hinders him. Another possibility is that an old, forgotten character from the Naruto universe is brought back and has some connection to the secret.