A 'adachi x nanako fanfic' could potentially explore the relationship between Adachi and Nanako. It might focus on their interactions, how they support each other, and the development of their friendship or a possible romantic connection. Maybe it shows how they grow together through shared experiences in their fictional world.
There could be. Fanfic writers love to explore different relationships and scenarios that aren't part of the original story. Given the popularity of both the Persona series (where Nanako Dojima is from) and Naruto, someone might have combined the two in a creative way in a fanfic.
The Jujutsu Kaisen manga is indeed ending. It's been a great run, but all good things must come to an end. The final chapters are bringing closure to the characters and story arcs.