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.
I'm not entirely sure without having watched it closely, but they are likely to be drug traffickers, law enforcement officers, and maybe some people caught in the middle like informants or family members of those in the drug trade.
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.