It could be a scientist. In the 20 years, he has been researching a new technology that could change the world. He faces many obstacles along the way, like lack of funding, rivalries with other scientists, but he perseveres. And in the end, his discovery has a huge impact on society, either for good or bad, depending on how the author wants to spin the story.
A detective who has been working on a cold case for 20 years and finally gets a breakthrough.
It could be an adventurer who has been exploring the world for 20 years and now returns home with countless stories.
It could be the original characters from the source material who have now aged 19 years. Their personalities might have changed a bit due to life experiences, but they still carry the essence of who they were.
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.
The children of Harry, Ron, and Hermione would likely be main characters. For instance, Harry's son could inherit his father's courage and be at the center of a new mystery. He might have to deal with the high expectations placed on him because of his father's legacy.
The main characters could be Ladybug and Chat Noir, who are now experienced heroes. However, the fanfiction could also focus on a new group of heroes that they are training. These new heroes could come from different backgrounds and have unique personalities. There might also be some characters from the past that have turned into antagonists due to some past grudges or misunderstandings. And of course, the Kwamis would still be there, guiding the new and old heroes alike.
Hermione Granger would be a likely main character. She has always been a central figure. Maybe she's working on some revolutionary magical research or is involved in political reforms in the wizarding world. Her relationship with Ron and their family life could also be part of the story.
Obviously Danny Phantom himself would still be a main character. His friends like Sam and Tucker would likely also be important. They've probably grown and developed new skills over the 10 years.
The main character could be a sentient energy form. Over 6 trillion years, it has been evolving and learning about the universe. However, overnight, it is forced to merge with a physical entity for some unknown reason. This new form causes all sorts of problems and adventures as it tries to adapt to its new state.
Maybe a music journalist who is researching a story about a particular genre of music over the past 70 years. The playlist is their guide, and they are the main character as they explore the significance of each song and its place in history. They meet different people related to the music along the way.
πI recommend Bald Grandmaster to you. It's a fantasy-Oriental fantasy novel. The protagonist was a bald blacksmith who left the cultivation world for eight years before returning to the secular world. He grew up, became stronger, became famous, and became rich, but he was still single. This is the seventh story of the author, Tian Shi. I believe you will like it very much. I hope you like this fairy's recommendation. Muah ~π