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.
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.
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.
There could also be new characters introduced. Maybe a business rival who is jealous of Naruto's success. This rival could be from another village or even an outsider who has his own motives for trying to take down Naruto's billion - dollar empire. And of course, the Hokage would still play a role, as Naruto's actions as a billionaire would have an impact on the village's politics and power structure.
The main characters would be Naruto and Axel. But there could also be other characters from the Naruto universe like Kakashi. Kakashi might be suspicious of Naruto's new abilities and try to figure out if they are a threat or not. And from Axel's side, his friends like Roxas could also make an appearance. They might initially be hostile towards Naruto but then learn to accept him as he proves himself.
In 'Naruto second Orochimaru fanfiction', Orochimaru is the central figure. Alongside him, Kabuto might be a prominent character as he was Orochimaru's right - hand man. Another character that could play a major role is Sakura. Since she is part of Team 7 with Naruto and Sasuke, and any threat or plot involving Orochimaru would likely draw her in. Also, there could be new characters created specifically for this fanfiction that interact closely with Orochimaru.
Naruto and Rias are bound to be the central characters. Naruto with his determination and power, and Rias with her leadership and magic. But we can't forget about the other members of Rias' peerage. They might have different reactions to Naruto being adopted. Some could be welcoming, while others might be a bit skeptical at first. And Naruto's own comrades from Konoha might also be relevant, perhaps in flashbacks or if they somehow cross over into Rias' world.