One idea could be exploring the backstories of the side characters more in - depth. For example, you could write about how Maki became so strong - willed. Another idea is to create an alternate universe where the curses and sorcerers have different power dynamics. Maybe the curses are the ones in control this time. And a third idea would be to focus on the relationships between characters outside of the main plot, like how Yuji and Todo's friendship could develop further in different scenarios.
First, you should be really familiar with the JJK universe. Read the manga, watch the anime. Then, pick a character or a group of characters you want to focus on. For example, if you like Gojo, you can start thinking about a story centered around him. Next, come up with a simple plot idea, like Gojo going on a solo mission to defeat a powerful curse. After that, start writing an outline of the story, including the beginning, middle and end.