Definitely Naruto himself. He would be an important character as the story is about his return after 20 years. His son or daughter could also be main characters, as they might be involved in the events related to Naruto's return.
A new character could be the main antagonist. This character could be from a far - off land that Naruto has never encountered before. This antagonist has heard of Naruto's power and wants to prove himself by defeating Naruto and taking over the Hidden Leaf. He might have unique powers and a large army at his disposal. He could also be manipulating some of the villagers from the inside to create chaos before making his big move.
Well, it could be about Naruto's kids facing a new threat that was somehow related to the past. Maybe an old enemy's apprentice has come back to seek revenge for their master's defeat decades ago.
Obviously, Naruto himself would still be a main character, as he is now an important figure in the village. His son Boruto could also be a main character, as he might be following in his father's footsteps. And Sasuke would likely still play a significant role, perhaps as a mentor to the new generation.
Obviously, Minato and Naruto are main characters. Minato is the one who returns, and Naruto is his son, so their relationship is central.
It could be a rogue ninja from a neighboring village who has a vendetta against Konoha. This ninja might see Naruto's return as an opportunity to take on the strongest ninja in Konoha and prove his own strength.
Obviously, Naruto himself would be one of the main characters. His wife, for example Hinata, would also be a main character as she is part of his family. And their children, like Boruto and Himawari, would surely play important roles too.
Aside from Naruto and his immediate family, characters like Kakashi could also be important in this fanfiction. He was Naruto's mentor, and he would surely be there to welcome them back. And Sasuke might also play a role, as he is Naruto's friend and has a connection to the village too.
The main characters would surely include Naruto. Since he's the one returning to his world. His friends from the village, like Shikamaru who is smart and could be involved in strategizing when Naruto comes back. And Hinata, as she has a special relationship with Naruto. Also, villains or antagonists that Naruto has to face or reconcile with could also be important characters in the fanfic.
There might be a dragon rider as one of the main characters. If the dragon had a rider in the past, perhaps that person or their descendant is involved in the story when the dragon returns. Another possible main character could be a villain who wants to use the dragon's power for evil purposes.
In a '10 year later returns naruto fanfic', Naruto could return to a village that has advanced technologically in ways he didn't expect. The shinobi world has started to incorporate more modern technology into their fighting styles. Naruto has to adapt to this new environment while also dealing with the fact that some of his old comrades are missing. He begins a search for them and discovers that they were kidnapped by a group who wants to use their powers for their own evil purposes. Along the way, he meets new characters who help him in his journey to rescue his friends and save the village once again.