Megumi Fushiguro is popular as well. His cool and reserved demeanor, along with his powerful cursed technique, attracts a lot of fanfiction authors. They might write about his relationship with other characters, like his unspoken bond with Yuuji, or explore how he further hones his skills in different scenarios.
One interesting concept is exploring different cursed techniques for the characters. For example, in some JJK fanfictions, Yuuji might have a cursed technique that allows him to communicate with the souls of the dead in a more detailed way. Another concept could be alternate universes where the characters have different relationships or roles. In some fanfictions, Megumi could be the main villain instead of an ally, which completely changes the dynamic of the story.
In a fanfiction, the plot could revolve around Yuji and Nobara being sent to a different dimension. In this dimension, the rules of jujutsu are different. They have to adapt quickly and find a way back home. During their time there, they meet strange versions of their friends and enemies. They must use their wits and powers to survive and figure out how to return to their own world. This experience changes their relationship and makes them stronger as a team.
One idea could be a story where Yuji and Nobara go on a special mission together in an unknown location filled with new types of curses. They have to rely on each other's strengths more than ever.
Yuji Itadori is often a popular choice. His determination and unique abilities make for interesting time - travel scenarios. He could use his physical strength and Sukuna's power in different ways in the past or future.