Todo's appearances in the JJK manga are scattered. It depends on the specific storylines and conflicts. Sometimes he's in the foreground, and other times he makes cameos to add to the overall narrative.
Yuji is still kicking in the JJK manga. The plot has thrown many challenges his way, but death hasn't claimed him yet. Who knows what the future holds though!
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.