The distance between the JJK manga and anime isn't too significant. The anime mostly adheres to the main plotlines of the manga, but there might be some minor tweaks for the animation format.
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 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.
The JJK anime lags behind the manga to a certain extent. There are new storylines and character growth in the manga that haven't made it to the anime screen yet.