Yuji Itadori could still be the main character. He might have developed new and more powerful techniques in this fanfiction, and be the one leading the charge against new threats in the future of the Jujutsu world.
Maybe in the fanfiction, the story focuses on a character from the future coming back to the present in 'Jujutsu Kaisen Watch the Future'. This character has knowledge of future events that could either save or destroy the current world of Jujutsu sorcerers. The main characters then have to decide whether to trust this person and how to use the information without causing chaos.
Yes, it's quite normal. Fanfiction often offers unique takes on the 'Jujutsu Kaisen' universe, and it can enhance the viewing experience. It allows fans to explore different storylines and character developments that might not be in the official show.
First, you need to have a good understanding of the Jujutsu Kaisen universe. Know the characters, their powers, and the rules of the jujutsu world. Then, decide on the character you want to reincarnate as or the new character you'll create for the reincarnation. Next, think about the plot. Will they be on a mission to change something in the story? Or will they just try to survive in this new world? Start writing, and don't be afraid to add your own twists.
The Jujutsu Kaisen manga is indeed ending. It's been a great run, but all good things must come to an end. The final chapters are bringing closure to the characters and story arcs.